bugfix for `view.py numInvIfgram.py` for `hyp3` products

[yunjunz]

Description of proposed changes

• ifgram_inversion: ignore the NO_DATA_VALUE metadata from the hyp3 ifgram stack file, while creating the # of inverted ifgram file, because 1) the zero value is valid (not no-data-value), and 2) to avoid the strange plotting error from matplotlib. The detailed reason seems to be within matplotlib, not within mintpy code itself.

• tests/configs/RidgecrestSenDT71: turn back ON the plotting during testing.

Reminders

• [x] Fix #1185
• [x] Pass Pre-commit check (green)
• [x] Pass Codacy code review (green)
• [x] Pass Circle CI and local tests (green)
• [x] Make sure that your code follows our style. Use the other functions/files as a basis.
• [x] If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.

[forrestfwilliams]

Thanks for this @yunjunz!

[yunjunz]

I think the code is now for a new release, will try to do it in the coming few days. Cheers!