Avoid `elementwise comparison failed` warning
Avoid `elementwise comparison failed` warning

If you have an array as an attribute, you get the warning:
```
nwb/find_links.py:1388: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  eq = x==y
```
when it compares the array to the empty string. This happens for instance
with the `starting_frame` attribute for an ImageSeries stored in external
files.

Fixed by adding an explicit test for the case where we compare a non-string
to a string. Note that `numpy.string_` is an instance of `str` so that
comparison still goes to the `x==y` clause.

NeurodataWithoutBorders/api-python,NeurodataWithoutBorders/api-python,NeurodataWithoutBorders/api-python