Hardening: add null guards and bounds checks to string, array, and table setters

[anagnorisis2peripeteia]

Summary

• Defensive hardening in include/flatbuffers/string.h, include/flatbuffers/array.h, and include/flatbuffers/table.h.
• In include/flatbuffers/string.h:
  • Return empty string literals when Data() is null in c_str().
  • Guard std::string and string_view construction against null buffers and zero size.
  • Guard operator< against null string buffers.
• In include/flatbuffers/array.h:
  • Guard Get() and GetMutablePointer() with out-of-bounds checks and null buffer guards.
  • In CopyFromSpan(): guard against null span and null destination data, and scale overlap check by sizeof(T).
  • In MutateImpl(): guard against out-of-bounds indices and null buffers.
• In include/flatbuffers/table.h:
  • In SetPointer(): guard against null pointer arguments and validate that pointer offset is non-negative (val >= data_ + field_offset).
• Add tests in tests/test.cpp covering all hardened paths.

[anagnorisis2peripeteia]

Closing this pull request.

These checks sit in accessors or helpers that run after verification. FlatBuffers checks an untrusted buffer in the verifier, once. A caller mistake is FLATBUFFERS_ASSERT. Returning 0, nullptr, false, or an empty value hides the mistake and changes behavior for a valid buffer.

See the C++ guide section "Access of untrusted buffers" and the review on #8990.