Handle singular data object in Vega model

[philippjfr]

Fixes bug where Vega data containing a singular data object would not get rendered and raise an error.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@f7fe0bb). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1201   +/-   ##
=========================================
  Coverage          ?   87.63%           
=========================================
  Files             ?      106           
  Lines             ?    12588           
  Branches          ?        0           
=========================================
  Hits              ?    11031           
  Misses            ?     1557           
  Partials          ?        0           

Impacted Files	Coverage Δ
panel/pane/equation.py	80.39% <0.00%> (ø)
panel/tests/pane/test_ace.py	100.00% <0.00%> (ø)
panel/tests/models/test_deckgl.py	100.00% <0.00%> (ø)
panel/callbacks.py	30.43% <0.00%> (ø)
panel/tests/pane/test_holoviews.py	99.46% <0.00%> (ø)
panel/tests/test_docs.py	100.00% <0.00%> (ø)
panel/tests/test_io.py	100.00% <0.00%> (ø)
panel/models/plotly.py	100.00% <0.00%> (ø)
panel/tests/test_cli.py	100.00% <0.00%> (ø)
panel/widgets/base.py	82.92% <0.00%> (ø)
... and 96 more

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