fix(theme): merge partial with empty initial partial

[nickofthyme]

Summary

Fix theming override logic to allow initial partial value as empty object ({}).

Before something like the chart below would ignore the dash definition because the first partial is empty.

export const Example = () => {
  return (
    <Chart>
      <Settings
        baseTheme={useBaseTheme()}
        theme={[
          {},
          {
            crosshair: {
              line: {
                dash: [4, 4],
              },
            },
          },
          EUI_CHARTS_THEME_LIGHT.theme,
        ]}
      />
    </Chart>
  );
};

Now the dash is correctly shown.

Details

The mergePartial logic had two issues...

1. The merge optional values logic only ran if the first partial was not undefined. Now it checks for any keys on all partial values at the given level and applies mutations to the baseClone as needed, if either the partial has applicable keys or any additionalPartials have applicable keys.
2. The final mapping over the new baseClone was only looking at the keys of the base and not the baseClone.

Unrelated, deprecates mergeWithDefaultTheme and removes all internal usage.

Issues

Actually fixes #1284

Related to elastic/kibana#116754

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• All related issues have been linked (i.e. closes #123, fixes #123)
• New public API exports have been added to packages/charts/src/index.ts
• Unit tests have been added or updated to match the most common scenarios

[markov00]

I tested the fix locally and works correctly when merging an empty object as the first partial object.
I think this actually doesn't completely fix the issue in Kibana, and it needs to go hand in hand with: #1453 and #1447

We need to backport those PRs into 38.0.x for Kibana 7.16

[nickofthyme]

🎉 This PR is included in version 38.1.4 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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