Fix ApplyLayout with empty layout (backport #14606)

[mergify]

Summary

It's unlikely to ever be useful, but strictly there still is a well-defined behaviour for ApplyLayout on a zero-qubit circuit and backend. Previously, the pass errored out falsely asserting user error.

Details and comments

---

This is an automatic backport of pull request #14606 done by [Mergify](https://mergify.com).

[qiskit-bot]

Thank you for opening a new pull request.

Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[kevinhartman]

Looks like this is failing due to an actual issue:

test.python.transpiler.test_apply_layout.TestApplyLayout.test_empty_post_layout
-------------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/Users/runner/work/qiskit/qiskit/test/python/transpiler/test_apply_layout.py", line 64, in test_empty_post_layout
    self.assertEqual(out.layout.initial_virtual_layout(), Layout())
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    AttributeError: 'NoneType' object has no attribute 'initial_virtual_layout'

[jakelishman]

Kevin: it's probably because the test is written under the assumption that #13820 was available. The tests probably need rewriting to instead construct a PassManager with an explicit SetLayout/whatever call followed by an ApplyLayout, and run that.

[coveralls]

Pull Request Test Coverage Report for Build 16980897218

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 20 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.01%) to 88.505%

---

Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/unitary_synthesis.rs	1	91.83%
crates/qasm2/src/expr.rs	1	93.78%
crates/qasm2/src/lex.rs	6	91.24%
crates/qasm2/src/parse.rs	12	97.1%

Totals
Change from base Build 16975764228:	-0.01%
Covered Lines:	77324
Relevant Lines:	87367

---

💛 - Coveralls