[bugfix] form_data - fix empty metrics

[timifasubaa]

This change introduced a regression (#5026)

Sometimes metrics can be set to None and the function assumes wrongly that if the metrics variable is specified, it must be a list. This PR fixes the regression.

@john-bodley @michellethomas

[codecov-io]

Codecov Report

Merging #5190 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #5190   +/-   ##
=======================================
  Coverage   77.46%   77.46%           
=======================================
  Files          44       44           
  Lines        8730     8730           
=======================================
  Hits         6763     6763           
  Misses       1967     1967

Impacted Files	Coverage Δ
superset/viz.py	81.29% <100%> (ø)	⬆️

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[rzlmma]

I'm more inclined to the last version, because the following code determines that it's value is an iterator.