Remove redundant null-handling for Title::newFromText

Remove redundant null-handling for Title::newFromText

* Linker: Follows-up 9ce5ca9886. null is already handled.

* PrefixSearch: Follows-up 337b1e2e. null is already handled.
  If other types are bassed, we want to find these.

* UploadBase: Follows-up 11f5ebec28. null is already handled.
  WebRequest::getVal($name) can only return string or null.

Change-Id: I3dc3b700cde735f5cd9d497f0867a2b4e3b61f46