Address `FutureWarning` in pyam/core.py:931

[phackstock]

Continuing along the lines of pyam warnings, I have found this one:

...pyam/core.py:931: FutureWarning: The behavior of array concatenation with empty entries is deprecated. In a future version, this will no longer exclude empty items when determining the result dtype. To retain the old behavior, exclude the empty entries before the concat operation.
  self.meta[name] = meta[name].combine_first(self.meta[name])