[ts] Builder incorrectly serializing empty strings (#7047)
[ts] Builder incorrectly serializing empty strings (#7047)

* [ts] Builder incorrectly serializing empty strings

The builder was returning an offset of zero for empty strings. This is
leading to flatbuffers which fail verification in other languages, such
as Rust.

* tests expect 0 offset for null or undefined strings
google/flatbuffers,alexames/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers,google/flatbuffers,google/flatbuffers,google/flatbuffers