Fix tapering of zero/empty operators

[woodsp-ibm]

Tapering logic would detect an empty/zero operator, log a warning and return the original operator unchanged. When running code with symmetry reduction in Qiskit Nature this ended up failing in VQE if one or more of the auxilary operators was zero. The set that ended up getting passed in would tapered non-zero operators and any zero operators were the originals which now were bigger, in terms of num_qubits, that the tapered operators. Given the number of qubits mismatched VQE raised an error.

This alters the code to so that a zero operator of the correct tapered size is returned from taper and not the original as before.

[woodsp-ibm]

@ikkoham - since I cannot add you as a reviewer...

[coveralls]

Pull Request Test Coverage Report for Build 1463550680

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 82.538%

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Totals
Change from base Build 1463415742:	0.003%
Covered Lines:	49807
Relevant Lines:	60344

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💛 - Coveralls

[ikkoham]

Thank you. I did not understand this part when I implement this. LGTM.