Allow zero-sized strings

[CalMorris]

Fixes #878

Summary

The FPP spec and the published website describe string size n as valid in the range [0, 2^31), but the compiler was rejecting string size 0 with an "invalid string size" error.

This is useful when a configured constant drives the string size and a value of 0 is used to disable a field.

Changes

• Relax the validation in FinalizeTypeDefs.scala from size > 0 to size >= 0
• Update the spec (Types.adoc) to document the valid range as [0,2^31-1]

Testing

Ran the existing fpp-check type test suite — all 7 tests pass, including string_size_negative (rejects -1) and string_size_too_large (rejects 0x80000000).

[bocchino]

Thanks for making this PR! In order to accept this change, we will need testing, to make sure that the zero-length string case works as expected in every case. I think we will need the following:

1. Additional tests here to test string size 0 and make sure that everything compiles: https://github.com/nasa/fpp/tree/main/compiler/tools/fpp-to-cpp/test.
2. Additional tests here to test string size 0 and make sure that everything works as expected in F Prime: https://github.com/nasa/fprime/tree/devel/FppTestProject/FppTest.
3. Fixes to any issues uncovered in these tests.

Ideally we would supplement every string size n test that we have with a corresponding string size 0 test. Is this something you would be willing to work on?