Emit the null merge patch when the target is JSON null

[youdie006]

MergePatch and MergePatchJSON return no patch at all when the target is JSON null, where
RFC 7386 says the patch is null.

patch := mergePatch(si, ti)
if patch == nil {
    return nil, nil
}
return json.Marshal(patch)

The guard reads as "there were no differences", but mergePatch never returns nil for that.
Two equal objects give {}, equal scalars give the scalar. merge.go:49 is its only nil
source:

if src == nil || tgt == nil {
    return tgt
}

So patch == nil holds in exactly one case - the target is null - which is the one case the
guard must not swallow. On master:

MergePatchJSON({"a":"foo"} , null   ) = ""        <- want `null`
MergePatchJSON(null        , null   ) = ""        <- want `null`
MergePatchJSON({"a":"foo"} , "bar"  ) = "bar"
MergePatchJSON({"a":1}     , {"a":1}) = "{}"      <- "no differences" is {}, not nil

An empty patch means "apply nothing", so a caller round-tripping {"a":"foo"} through the patch
keeps {"a":"foo"} where the RFC requires null.

This is already asserted by the repo's own test data - testdata/tests/mergepatch/rfc.json case
#11, {"a":"foo"} -> null => null, straight from RFC 7386 Appendix A. It passes because
merge_test.go:36 calls the unexported mergePatch directly and never goes through the exported
wrappers. The public surface of merge-patch generation is covered only by the two happy-path
examples in example_test.go.

The change

Drop the guard from both wrappers - json.Marshal(nil) already emits null.

Observable change, stated plainly: for a null target these now return []byte("null") and
nil error, where before they returned nil, nil. Anything treating an empty result as "no
change" will see 4 bytes instead. That is the point of the fix, but it is a behaviour change on
that one input. Every other input is byte-identical.

Test

Test_mergePatchNullTarget in merge_test.go, driving MergePatchJSON rather than the
unexported function, with RFC #12 alongside as a control. Reverting only merge.go:

--- FAIL: Test_mergePatchNullTarget/rfc_#11
    merge_test.go:65: got "", want "null"
--- FAIL: Test_mergePatchNullTarget/null_to_null
    merge_test.go:65: got "", want "null"

rfc #12 passes either way, confirming the non-null path is untouched.

CI's own command passes with the change:

go test -race -coverprofile=coverage.txt -covermode=atomic ./...
ok  github.com/wI2L/jsondiff  1.184s  coverage: 97.3% of statements

go vet and gofmt -l clean.

---

Disclosure: found and prepared with AI assistance (Claude). Every figure above is from a run on
this branch.