Ensure DataFrame with no index is editable

[philippjfr]

Fixes #1534

[codecov]

Codecov Report

Merging #1547 into master will increase coverage by 0.01%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##           master    #1547      +/-   ##
==========================================
+ Coverage   85.60%   85.61%   +0.01%     
==========================================
  Files         146      146              
  Lines       16161    16179      +18     
==========================================
+ Hits        13834    13852      +18     
  Misses       2327     2327              

Impacted Files	Coverage Δ
panel/widgets/tables.py	89.95% <50.00%> (ø)
panel/models/__init__.py	100.00% <100.00%> (ø)
panel/models/widgets.py	100.00% <100.00%> (ø)
panel/widgets/__init__.py	100.00% <100.00%> (ø)
panel/widgets/select.py	93.24% <100.00%> (+0.31%)	⬆️

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