Properly check whether dataframe is empty

[andersy005]

Change Summary

Fixes valueError when checking whether the dataframe is empty

--> 323         esmcat_results = self.esmcat.search(require_all_on=require_all_on, query=query)
    324 
    325         # step 2: Search for entries required to derive variables in the derived catalogs

/glade/work/abanihi/devel/intake/intake-esm/intake_esm/cat.py in search(self, query, require_all_on)
    316             df=self.df, query=_query.query, columns_with_iterables=self.columns_with_iterables
    317         )
--> 318         if _query.require_all_on is not None and results:
    319             results = search_apply_require_all_on(
    320                 df=results, query=_query.query, require_all_on=_query.require_all_on

/glade/work/abanihi/opt/miniconda/envs/playground/lib/python3.8/site-packages/pandas/core/generic.py in __nonzero__(self)
   1535     @final
   1536     def __nonzero__(self):
-> 1537         raise ValueError(
   1538             f"The truth value of a {type(self).__name__} is ambiguous. "
   1539             "Use a.empty, a.bool(), a.item(), a.any() or a.all()."

ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

Related issue number

Checklist

• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable