Add condition in WorldMapViz before using Pandas' iloc

[elukey]

Recent pandas versions (0.23+) changed the value of df[metric]
when the same metric is used in a World Map panel for both bubble size
and an axis (either X or Y). This change adds some guards before
using panda's iloc to avoid exceptions.

Should fix #7006

[codecov-io]

Codecov Report

Merging #7156 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #7156      +/-   ##
==========================================
- Coverage   64.63%   64.63%   -0.01%     
==========================================
  Files         422      422              
  Lines       20607    20609       +2     
  Branches     2253     2253              
==========================================
  Hits        13320    13320              
- Misses       7165     7167       +2     
  Partials      122      122

Impacted Files	Coverage Δ
superset/viz.py	71.44% <0%> (-0.09%)	⬇️

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