Fix BitArray.from_counts/from_samples to not fail for input with only 0 outcome and num_bits=None (backport #12800)

[mergify]

Summary

Partial fix for #12765.

That issue points out two problems with BitArray.from_counts / BitArray.from_samples. The more severe problem is that these methods fail with an error if the input (e.g. a counts dictionary) contains only the 0 outcome, and num_bits is left at the default value of None.

The methods try to infer num_bits as the number of bits needed to represent the largest outcome (e.g. bitstring) contained in the input data. If the largest outcome is 0 when converted to an integer (e.g. the bitstring "0"), the number of bits inferred is 0, which can be surprising, and also leads to an error.

This small PR interprets 0 as represented by 1 bit instead of by 0 bits, the suggested correct behavior in #12765.

The less severe problem, not fixed here, is that currently any information about num_bits present in the input bitstring lengths gets ignored. That problem seemed less trivial to me. I put some thoughts here.

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This is an automatic backport of pull request #12800 done by [Mergify](https://mergify.com).

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[coveralls]

Pull Request Test Coverage Report for Build 10109349540

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 14 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.001%) to 89.944%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/transpiler/passes/synthesis/unitary_synthesis.py	2	88.25%
crates/qasm2/src/lex.rs	6	92.11%
crates/qasm2/src/parse.rs	6	97.15%

Totals
Change from base Build 10091438649:	-0.001%
Covered Lines:	61851
Relevant Lines:	68766

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💛 - Coveralls