[BUGFIX] Fix SQL error in Livesearch with empty TCA sortby setting

If a searchable TCA field has a sortby setting which contains an empty
string or NULL, then BE Livesearch creates a query with one or more
'ORDER BY' without value, generating an SQL error. This fixes that.

Change-Id: I0457020ef31850458e3bd4286214f8e57278223e
Fixes: #40816
Releases: 6.0, 4.7, 4.6, 4.5
Reviewed-on: https://review.typo3.org/15541
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Reviewed-by: Mario Rimann
Reviewed-by: Steffen Ritter
Reviewed-by: Dmitry Dulepov
Tested-by: Dmitry Dulepov
Reviewed-by: Ernesto Baschny