Fix execute_circuit to handle 0 num_qubits

[guan404ming]

Purpose of PR

This PR prevents errors that would occur when trying to execute an empty circuit

Linked Issues

Add links to related issues.

• Related to Refactor create_empty_circuit to handle num_qubits #586

Changes Made

• Bug fix
• New feature
• Documentation update

Important ToDos

Please mark each with an "x"

A GitHub issue exists (if not, please create one) [https://github.com/apache/mahout/issues]

• Title of PR is "Issue #XXXX: Brief Description of Changes" where XXXX is the GitHub issue number.
• Created unit tests where appropriate
• Added correct licenses on newly added files
• Assigned GitHub issue to self
• Added documentation in ScalaDocs/JavaDocs and to the website
• Successfully built and ran all unit tests, verified that all tests pass locally

If all of these items are not yet complete, but you still feel it is appropriate to open a PR, please open it as a Draft PR instead.
Once all requirements are met, you can mark it as ready for review.

Breaking Changes

Does this PR introduce a breaking change?

• Yes
• No

Testing & Verification

Describe how you tested the changes.

• Unit tests added
• Manually tested

Checklist

• The title follows the format "MAHOUT-XXXX Brief Description"
• GitHub issue is created
• Code follows ASF guidelines

[andrewmusselman]

Looks good; thanks @guan404ming

[andrewmusselman]

@guan404ming let's fix the failing tests before merging; if this is reliant on something already merged we could possibly go for it