Handle empty ProForma formulas

[BioGeek]

This PR fixes an existing mzcore ProForma molecular formula parser panic for empty input.

Previously, MolecularFormula::pro_forma::<_, true>("") could attempt an inclusive slice on an empty string and panic, even though empty formulas are documented as valid when ALLOW_EMPTY is enabled.

Changes:

• Detect empty input before slicing the requested formula range.

• Preserve existing behavior:

  • empty formulas return Ok(default) when ALLOW_EMPTY = true
  • empty formulas return an error when ALLOW_EMPTY = false

• Add regression coverage for both accepted and rejected empty input.

Validated xith:

CARGO_INCREMENTAL=0 cargo test -p mzcore --doc MolecularFormula::pro_forma --no-fail-fast
CARGO_INCREMENTAL=0 cargo test -p mzcore pro_forma_empty --lib --no-fail-fast

Both pass.

[douweschulte]

Thanks for the fix. I think I inadvertently broke this recently when cleaning up this region of code.