[Dart]fix: segment fault with empty namespace when generating dart file (#5507)
[Dart]fix: segment fault with empty namespace when generating dart file (#5507)

* fix: segment fault with empty namespace when generating dart file

* fix: compile err

* fix: compile err

* fix: imported file names

* fix test issues

* fix some language style issues

google/flatbuffers,pjulien/flatbuffers,pjulien/flatbuffers,DavadDi/flatbuffers,pjulien/flatbuffers,google/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,DavadDi/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,pjulien/flatbuffers,pjulien/flatbuffers,alexames/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,google/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers,google/flatbuffers,google/flatbuffers,evolutional/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,pjulien/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers,pjulien/flatbuffers,DavadDi/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,google/flatbuffers,google/flatbuffers,pjulien/flatbuffers,google/flatbuffers,pjulien/flatbuffers,DavadDi/flatbuffers,evolutional/flatbuffers,google/flatbuffers,DavadDi/flatbuffers,pjulien/flatbuffers,google/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,google/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,evolutional/flatbuffers