Bug 29077: (follow-up) Use another syntax

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

Koha/SearchEngine/Elasticsearch/QueryBuilder.pm

@@ -714,7 +714,7 @@ sub _convert_index_fields {
     my %index_type_convert =
       ( __default => undef, phr => 'phrase', rtrn => 'right-truncate', 'st-year' => 'st-year' );
 
-    @indexes = grep(/.+/, @indexes); # Remove any blank indexes, i.e. keyword
+    @indexes = grep { $_ ne q{} } @indexes; # Remove any blank indexes, i.e. keyword
 
     # Convert according to our table, drop anything that doesn't convert.
     # If a field starts with mc- we save it as it's used (and removed) later