fix channel label mismatch

[will-moore]

If multiple images with same number of channels are selected, the channel buttons previously showed "undefined" if the labels didn't match.
Now we show no text.

Fixes #364.

Before:

After:

[jburel]

The blank button should be shown if the name of the channel does not match
if for example we have 2 images with both DAPI then DAPI should be used instead of a blank button

[will-moore]

That should be how it's working already?

e.g. 2 image selected - First channel is named differently but the other channels have same name in both images:

[jburel]

I thought so, I was a bit confused by the description in the PR

[jburel]

[jburel]

[jburel]

Looks good