combine_first doesn't fill empty strings

[jpdehollain]

What happened?

If a data variable elements are string types, the default empty value is an empty string ''. When I use combine_first to try to fill some empty strings in a data variable, the method doesn't fill them.

What did you expect to happen?

Elements with empty strings should be filled with non-empty strings from the other dataset.

Minimal Complete Verifiable Example

ds1 = xr.DataArray(['a','',''], dims='dim', coords={'dim': [1,2,3]})
ds2 = xr.DataArray(['b'], dims='dim', coords={'dim': [2]})
ds1.combine_first(ds2)

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Anything else we need to know?

No response

Environment

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