Fix Target.instruction_supported when target.num_qubits == None (backport #13655)

[mergify]

Summary

This PR fixes a small bug found in Target.instruction_supported, where target.num_qubits == None would cause Target.instruction_supported to always evaluate to False. This was handled in the original Python implementation by artificially overwriting qargs, but this would require a mutable variable in Rust, so I just added an additional condition to account for this case.

Details and comments

Bug discovered through #12850.

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This is an automatic backport of pull request #13655 done by [Mergify](https://mergify.com).

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[coveralls]

Pull Request Test Coverage Report for Build 12753048562

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 21 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 88.916%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	9	91.98%
crates/qasm2/src/parse.rs	12	97.15%

Totals
Change from base Build 12709260875:	-0.02%
Covered Lines:	79154
Relevant Lines:	89021

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💛 - Coveralls