Honor exception: false for strings with null bytes

[OskarEichler]

Summary:

• detect embedded NUL bytes before StringValueCStr in non-raising conversion mode
• return nil when exception: false
• retain the existing ArgumentError when exceptions are enabled

Why:
StringValueCStr raises string contains null byte before BigDecimal’s parser can honor exception: false. Core Float and Integer conversions use their non-raising option for this case, and BigDecimal already documents the same conversion mode.

Verification:

• baseline reproduction raises for BigDecimal("1\0", exception: false)
• fixed reproduction returns nil; exception: true still raises; ordinary strings are unchanged
• full current suite: 266 tests, 13,616 assertions, 0 failures/errors/omissions
• RBS suite: 146 tests, 1,805 assertions, 0 failures/errors

Compatibility:
Only strings invalid because of an embedded NUL change in the explicitly non-raising mode.