Fix more spurious complex parts in circuit functions (backport #14431)

[mergify]

Summary

@ElePT reported that #13643 came back for other circuits, too. This essentially affects circuits that use operators as inputs, which have coefficients of complex types. This problem does not appear on main, I assume #13278 removes the 0 complex component. We could still consider to add this safetyguard to main, but there's no way to break it right now that I'm aware of.

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This is an automatic backport of pull request #14431 done by [Mergify](https://mergify.com).

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[Cryoris]

The tests added here only trigger the issue prior to #13278 -- so they don't actually check the changed behavior on main. We should add a test to explicitly check the fix, probably via the QPY compatibility checks where we dump a circuit and load it with v2.0 or before?

[coveralls]

Pull Request Test Coverage Report for Build 15270397280

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 18 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.01%) to 88.323%

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Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/symbol_expr.rs	1	75.02%
crates/qasm2/src/expr.rs	1	94.23%
crates/qasm2/src/lex.rs	4	92.73%
crates/qasm2/src/parse.rs	12	97.15%

Totals
Change from base Build 15188294429:	-0.01%
Covered Lines:	78467
Relevant Lines:	88841

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💛 - Coveralls