Fixed new non-text calculations output a missing-label warning

[gushil]

Closes #
#451

Why is this the best possible solution? Were any other approaches considered?

The simplest one.

What are the regression risks?

None.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

None.

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests_v1
• run nosetests and verified all tests pass
• run black pyxform to format code
• verified that any code or assets from external sources are properly credited in comments

[gushil]

@MartijnR I haven't added any test cases for this issue yet because I couldn't find a way to test warning yet. Do you have any suggestions about that?

Thanks

[codecov-commenter]

Codecov Report

Merging #458 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #458   +/-   ##
=======================================
  Coverage   83.11%   83.11%           
=======================================
  Files          25       25           
  Lines        3595     3595           
  Branches      833      833           
=======================================
  Hits         2988     2988           
  Misses        460      460           
  Partials      147      147           

Impacted Files	Coverage Δ
pyxform/xls2json.py	78.00% <ø> (ø)

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[MartijnR]

Thanks! I'll do some testing. Wrt to testing for warnings, see example here: https://github.com/XLSForm/pyxform/blob/master/pyxform/tests_v1/test_dynamic_default.py#L310-L316

[MartijnR]

I did not find issues with the functionality.

[gushil]

@MartijnR I'm adding some tests.

Thanks.