Remove invalid date from banned delegate warning

[isalga]

Expected behavior

Should display a valid date or nothing, not 'Invalid date'.

Actual behavior

Displays invalid date

Steps to reproduce

• Go to a banned delegate page and observe the warning

Additional information

Once delegate is banned Lisk Service removes pomHeights resulting in the date being impossible to calculate.

Which version(s) does this affect? (Environment, OS, etc...)

mac, v2.2.0-beta.0