Describe the bug
When using a theme override in certain circumstances the first values do NOT take precedence and revert to the baseTheme value.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/happy-violet-xcb3z?file=/src/App.tsx

Expected behaviour
In the example codesandbox above, I expect the crosshair value to be applied from theme2 but it is actually being applied from the baseTheme.

Additional context
Likely related to the mergeOptionalPartialValues option on the mergePartial utility function.

Related to https://github.com/elastic/kibana/pull/106845/files/039ca4ef1a815cc9d76c9ebbad3ce7a00be26115..ac3eb6a10fbe552743d7a9075c104a2c010ea043