Update plisttool to handle empty input plists, to allow for routing through strings and plists safely.

Cherry-pick: 9eb4232

Author: nglevin

tools/plisttool/plisttool.py

@@ -774,6 +774,9 @@ def _read_plist(cls, plist_file, name, target):
       PlistToolError: if plutil return code is non-zero.
     """
     plist_contents = plist_file.read()
+    if not plist_contents:
+      # If the incoming file is empty, return an empty dictionary.
+      return {}
 
     # Binary plists are easy to identify because they start with 'bplist'. For
     # plain text plists, it may be possible to have leading whitespace, but

tools/plisttool/plisttool_unittest.py

@@ -459,6 +459,19 @@ def test_merge_of_one_dict(self):
     self._assert_plisttool_result({'plists': [plist1]}, {'Foo': 'abc'})
 
   def test_merge_of_one_empty_file(self):
+    plist_fp = tempfile.NamedTemporaryFile(delete=False)
+    self.addCleanup(lambda: os.unlink(path=plist_fp.name))
+    self._assert_plisttool_result({'plists': [plist_fp.name]}, {})
+
+  def test_merge_of_one_empty_file_with_binary_output(self):
+    plist_fp = tempfile.NamedTemporaryFile(delete=False)
+    self.addCleanup(lambda: os.unlink(path=plist_fp.name))
+    self._assert_plisttool_result({
+        'binary': True,
+        'plists': [plist_fp.name],
+    }, {})
+
+  def test_merge_of_one_empty_xml_file(self):
     plist1 = _xml_plist('')
     self._assert_plisttool_result({'plists': [plist1]}, {})