"cregbundle set to False" warning should not raised when no classical bits are involved

[1ucian0]

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@Guillermo-Mijares-Vilarino discovered that the following code raises a very confusing warning:

from qiskit.circuit.library import QFT
qft = QFT(3)
qft.decompose().draw('mpl', reverse_bits=True)

RuntimeWarning: Cregbundle set to False since either reverse_bits or wire_order has been set.
  wire_order=wire_order,

This fix checks if classical bits are involved before touching cregbundle.

Fixes #8690

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[coveralls]

Pull Request Test Coverage Report for Build 3162371682

• 12 of 12 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 84.663%

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Totals
Change from base Build 3162371342:	0.004%
Covered Lines:	61510
Relevant Lines:	72653

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💛 - Coveralls

[Guillermo-Mijares-Vilarino]

This PR would be very helpful for the explanation section I'm adding in #8685.

[jakelishman]

Would you like to make this part of a slightly larger issue surrounding the cregbundle parameter? I've just made #8690 to describe it, although the behaviour has been present for a super long time. Your fix here is good with or without touching that issue, I'm just bringing it up since you're touching the code anyway.

[1ucian0]

Would you like to make this part of a slightly larger issue surrounding the cregbundle parameter?

I fixed that one too by changing the way that the default is handled for that parameter.