Fix flow rate issue for fanless unitary system

Pull request overview

Fixes #10823
Change setpoint controlled unitary system with no fan to leave inlet mass flow rate as is.

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

[x] Title of PR should be user-synopsis style (clearly understandable in a standalone changelog context)
[x] Label the PR with at least one of: Defect, Refactoring, NewFeature, Performance, and/or DoNoPublish
[x] Pull requests that impact EnergyPlus code must also include unit tests to cover enhancement or defect repair
[ ] Author should provide a "walkthrough" of relevant code changes using a GitHub code review comment process
[ ] If any diffs are expected, author must demonstrate they are justified using plots and descriptions
[x] If changes fix a defect, the fix should be demonstrated in plots and descriptions
[x] If any defect files are updated to a more recent version, upload new versions here or on DevSupport
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This will not be exhaustively relevant to every PR.

[x] Perform a Code Review on GitHub
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[ ] If defect, verify by running develop branch and reproducing defect, then running PR and reproducing fix
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[ ] Check that performance is not impacted (CI Linux results include performance check)
[ ] Run Unit Test(s) locally
[ ] Check any new function arguments for performance impacts
[ ] Verify IDF naming conventions and styles, memos and notes and defaults
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