fix: [~] str interpolator now prints empty if null column (issue #191)

[eruizalo]

Description

Using concat_ws instead of concat we can avoid this error (but we have as limitation that nulls will be skipped)

Related Issue

Resolves #191

How Has This Been Tested?

• This pull request contains appropriate tests?

[codecov]

Codecov Report

Merging #199 (bea53bd) into main (ae94977) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #199   +/-   ##
=======================================
  Coverage   95.99%   95.99%           
=======================================
  Files          49       49           
  Lines         798      798           
  Branches        9       11    +2     
=======================================
  Hits          766      766           
  Misses         32       32           

Flag	Coverage Δ
spark-3.0.x	94.82% <100.00%> (ø)
spark-3.1.x	95.97% <100.00%> (ø)
spark-3.2.x	95.97% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...re/src/main/scala/doric/syntax/Interpolators.scala	100.00% <100.00%> (ø)

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