Fix the rendering of empty dataframes

[mwouts]

In this PR, we make sure that all the possible variations of empty dataframes (empty, no rows, no columns etc) are rendered correctly - previously on some of these empty dataframes we were seeing a message "loading" forever.

[codecov-commenter]

Codecov Report

Merging #184 (20eaf7a) into main (dbc66c1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
+ Coverage   97.29%   97.30%   +0.01%     
==========================================
  Files          24       25       +1     
  Lines         849      853       +4     
==========================================
+ Hits          826      830       +4     
  Misses         23       23              

Impacted Files	Coverage Δ
itables/sample_dfs.py	100.00% <ø> (ø)
itables/javascript.py	94.11% <100.00%> (+0.02%)	⬆️
itables/shiny.py	100.00% <100.00%> (ø)
itables/version.py	100.00% <100.00%> (ø)
tests/sample_python_apps/itables_in_a_shiny_app.py	100.00% <100.00%> (ø)
...le_python_apps/itables_in_a_shiny_app_with_tabs.py	100.00% <100.00%> (ø)
tests/test_documentation_notebooks_run.py	100.00% <100.00%> (ø)

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