[idl_parser, JSON] Disable parsing of JSON for incomplete schemes (#6493)
[idl_parser, JSON] Disable parsing of JSON for incomplete schemes (#6493)

This commit disable JSON parsing for an incomplete scheme if JSON object is embedded into one file with the scheme.
This should improve the quality of OSS-Fuzz inputs for the parser_fuzzer target.
alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers,DavadDi/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,evolutional/flatbuffers,evolutional/flatbuffers,DavadDi/flatbuffers,evolutional/flatbuffers,google/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,google/flatbuffers,evolutional/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,google/flatbuffers,google/flatbuffers,evolutional/flatbuffers,google/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers