[BUGFIX] indexed_search: in some cases indexing does not work

In some cases $pObj->register['SYS_LASTCHANGED'] is NULL and therefore
a MySQL error occurs during indexing a page. So, in these cases,
we need to set the 'mtime' value by getting it from the page and we
always want to prevent a NULL-value getting inserted into the DB.

Resolves: #61135
Releases: 6.1, 6.2, 6.3
Change-Id: Idb78af3aa23350db4bd43f58638541e6b3ad3ae3
Reviewed-on: http://review.typo3.org/32601
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>