Signalfillempty set track_meta to True

[matt3o]

Related to #7011. I was using the Signalfillemptyd (based on Signalfillempty) transform in monailabel and found out, it currently allows no inversion.
When digging deeper I realized that SignalFillEmpty just throws away the meta information.
With the simple addition of track_meta=True it works as expected.
I hope it has no other impact, but I honestly don't know.

@wyli would be cool if we can add this to MONAI 1.3.0, thanks! I can also rework Signalfillempty to just accept any datatype, if that would the more appropriate approach.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Documentation updated, tested make html command in the docs/ folder.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.

[wyli]

Thanks, it looks good to me.

[wyli]

/build

[wyli]

/build

[wyli]

/build