[Backport] Mapping Builder - hide Add button if there are empty selections

[mturley]

Backports #522

[konveyor-preview-bot]

🚀 Deployed Preview: http://konveyor-forklift-ui-pr-523-preview.surge.sh ✨

Compare with current main branch: http://konveyor-forklift-ui-preview.surge.sh

[codecov]

Codecov Report

Merging #523 (14abc90) into release-v2.0.0 (9b7ab21) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@               Coverage Diff               @@
##           release-v2.0.0     #523   +/-   ##
===============================================
  Coverage           59.73%   59.73%           
===============================================
  Files                 133      133           
  Lines                4304     4297    -7     
  Branches             1070     1069    -1     
===============================================
- Hits                 2571     2567    -4     
+ Misses               1709     1706    -3     
  Partials               24       24           

Impacted Files	Coverage Δ
...pings/components/MappingBuilder/MappingBuilder.tsx	76.19% <0.00%> (+1.90%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9b7ab21...14abc90. Read the comment docs.