Don't use isset() to check for null

Don't use isset() to check for null

- Remove isset for defined member variables
- Add a missing definition of member variable $mTextId
- Fix documentation of $mTextRow
- Standardize checks for null to use "=== null" or "!== null"
  instead of is_null()

Change-Id: I56e364bc14b5a3961a2538371ae4b0088babc5c7