frontend: Fix SearchableListModel.getSearchString()

Commit caef3ae [frontend: Add paging in SubTabPoolVmView] has
modified SearchableListModel.getSearchString() implementation
so that it returns "null" (String) instead of null (literal)
when searchString field is null.

In practice, GUI bound to SearchableListModel.searchString
via GWT Editor framework now shows "null" (String) instead
of empty string (due to null literal) when searchString field
is null, for example:

Data Center main tab | Permission sub tab | Add button
-> text box left to "GO" button now shows "null" string

This patch fixes the above mentioned problem by ensuring
getSearchString() returns null (instead of "null") when
searchString field is null.

Change-Id: I2fc2cab5785597cd7a7c9602a967250f16742489
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>