Update BitArray.get_counts() to support zero classical registers

[atharva-satpute]

Summary

Fixes #12062
Added support for zero classical registers in BitArray.get_counts(), BitArray.get_int_counts() and BitArray.get_bitstring()

Details and comments

1. With this change, if BitArray is initialised with zero classical registers it will return {'': <count>} for get_counts() and {0: <count>} for get_int_counts()
2. Noticed a case where loc is not None and arr is a number (arr = self._array[loc]), this would throw a TypeError on line, to handle this converted arr to np.array and along with this, checking arr.size == 0 in case get_bitstring()

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[coveralls]

Pull Request Test Coverage Report for Build 8979374846

Warning: This coverage report may be inaccurate.

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Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 781 unchanged lines in 42 files lost coverage.
• Overall coverage increased (+0.1%) to 89.638%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/converters/circuit_to_gate.py	1	97.44%
qiskit/circuit/classical/expr/constructors.py	1	91.82%
qiskit/primitives/backend_estimator_v2.py	1	98.29%
qiskit/circuit/controlflow/control_flow.py	1	92.86%
qiskit/transpiler/passes/layout/apply_layout.py	1	98.25%
qiskit/primitives/statevector_sampler.py	1	99.08%
qiskit/circuit/store.py	1	94.44%
crates/accelerate/src/sabre/mod.rs	1	75.0%
qiskit/converters/circuit_to_instruction.py	1	98.28%
qiskit/circuit/classical/expr/expr.py	1	93.51%

Totals
Change from base Build 8910124709:	0.1%
Covered Lines:	62256
Relevant Lines:	69453

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💛 - Coveralls

[ihincks]

Thanks for taking this on @atharva-satpute . I think we will also need to make sure that from_bitstrings, from_int also support the num_bits=0 case, because I suspect that they don't right now.

[atharva-satpute]

Sure

[ewinston]

@ihincks It seems BitString doesn't support from_bitstrings or from_int. The only reference I see to from_bitstrings is in the documentation of the DataBin class.

However, @atharva-satpute , I think it should probably support from_mapping({'', 1024})

[atharva-satpute]

What would from_mapping() method do?