[Rust] Use the schema's null value when generating enums

[s1amese2003]

Fixes #1116.

What

The Rust generator emits the primitive type's default null value for an enum
instead of the one declared in the schema. For a uint8 enum whose encoding
declares nullValue="254", it generates NullVal = 0xff_u8. Java, C++ and Go
all generate 254 for the same schema, so a Rust encoder writes a null a peer in
another language does not recognise.

Why

IrGenerator records the null value on the enum's BEGIN_ENUM token. The
VALID_VALUE tokens carry only byteOrder, primitiveType and constValue.
generateEnum read the null value from messageBody.get(0) — a valid value
token — so applicableNullValue() fell back to primitiveType.nullValue().
JavaGenerator reads it from tokens.get(0), which is why Java is correct.

The same mistake appeared twice, in the enum discriminant and in
From<Enum> for <primitive>. The null value is now resolved once and passed
down, so the two cannot disagree.

Checks

• ./gradlew runRustTests — 33 passed, 0 failed
• ./gradlew :sbe-tool:test — pass
• ./gradlew :sbe-tool:checkstyleMain :sbe-tool:checkstyleTest — pass

Regenerating every Rust codec in the repository before and after the change
leaves all 133 generated files byte-identical except the two lines above in the
new issue1116 codec.

optional_enum_nullify.xml already covers an enum with a declared null value,
but it declares 255, which is also the uint8 default, so it passes either way.
The new schema declares 254 to separate the two.