Fix unintended failure of if self._call_output in future.py for objects with ambiguous truth values

[cisaacstern]

Closes #1396

As described in #1396, in future.py, the check if self._call_output fails (unintentionally) if the object assigned to self._call_output has an ambiguous truth value, as is the case for pandas dataframes and numpy ndarrays.

Changing this check to if self._call_output is not None fixes this unintended failure. The added test checks for this behavior with a custom object, to avoid taking a pandas and/or numpy dependency in the test environment.

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[JosepSampe]

Thanks for the contribution @cisaacstern !