Fix the unexpected error that occurs when an empty control block is used.

Fixes： #146

When the first control block is empty or comment, the correct result will now be rendered.

```python
from mako.template import Template
template = r"""
% if False:
% elif False:
% else:
    test
% endif
"""

t = Template(template)
print(t.render())
```

Result:
```

    test

```

Closes: #387
Pull-request: #387
Pull-request-sha: fb257ff

Change-Id: Ief78cdf4eb39b6e8257332ed034dab052c715420

Author: cocolato

doc/build/unreleased/146.rst

@@ -0,0 +1,8 @@
+.. change::
+    :tags: bug, codegen
+    :tickets: 146
+
+    Fixed unexpected error when use control lines which the
+    first control block with no bodies other than comments,
+    as `pass` is now added to the first empty block.
+    Pull request courtesy Hai Zhu.

mako/codegen.py

@@ -838,13 +838,24 @@ def visitControlLine(self, node):
                 text = node.text
             self.printer.writeline(text)
             children = node.get_children()
-            # this covers the three situations where we want to insert a pass:
-            #    1) a ternary control line with no children,
-            #    2) a primary control line with nothing but its own ternary
-            #          and end control lines, and
-            #    3) any control line with no content other than comments
-            if not children or (
-                all(
+
+            # this covers the four situations where we want to insert a pass:
+            # 1) a ternary control line with no children,
+            # 2) a primary control line with nothing but its own ternary
+            #       and end control lines, and
+            # 3) any control line with no content other than comments
+            # 4) the first control block with no content other than comments
+            def _search_for_control_line():
+                for c in children:
+                    if isinstance(c, parsetree.Comment):
+                        continue
+                    elif isinstance(c, parsetree.ControlLine):
+                        return True
+                    return False
+
+            if (
+                not children
+                or all(
                     isinstance(c, (parsetree.Comment, parsetree.ControlLine))
                     for c in children
                 )
@@ -853,6 +864,7 @@ def visitControlLine(self, node):
                     for c in children
                     if isinstance(c, parsetree.ControlLine)
                 )
+                or _search_for_control_line()
             ):
                 self.printer.writeline("pass")
 

test/test_template.py

@@ -1036,6 +1036,47 @@ def test_blank_control_8(self):
             template_args={"ctx": ctx},
         )
 
+    def test_blank_control_9(self):
+        self._do_memory_test(
+            """
+            % if True:
+            % elif False:
+            false
+            % else:
+            broken
+            % endif
+            """,
+            "",
+            filters=lambda s: s.strip(),
+            template_args={"ctx": ctx},
+        )
+
+    def test_blank_control_10(self):
+        self._do_memory_test(
+            """
+            % if True:
+            % else:
+            test
+            % endif
+            """,
+            "",
+            filters=lambda s: s.strip(),
+            template_args={"ctx": ctx},
+        )
+
+    def test_blank_control_11(self):
+        self._do_memory_test(
+            """
+            % try:
+            % except:
+            error
+            % endtry
+            """,
+            "",
+            filters=lambda s: s.strip(),
+            template_args={"ctx": ctx},
+        )
+
     def test_commented_blank_control_1(self):
         self._do_memory_test(
             """
@@ -1135,6 +1176,36 @@ def test_commented_blank_control_8(self):
             template_args={"ctx": ctx},
         )
 
+    def test_commented_blank_control_9(self):
+        self._do_memory_test(
+            """
+            % if True:
+            ## comment
+            % elif False:
+            false
+            % else:
+            broken
+            % endif
+            """,
+            "",
+            filters=lambda s: s.strip(),
+            template_args={"ctx": ctx},
+        )
+
+    def test_commented_blank_control_10(self):
+        self._do_memory_test(
+            """
+            % try:
+            ## comment
+            % except:
+            error
+            % endtry
+            """,
+            "",
+            filters=lambda s: s.strip(),
+            template_args={"ctx": ctx},
+        )
+
     def test_multiline_control(self):
         t = Template(
             """