FIJISettingsViewer: allow FIU names to be replaced/deleted by empty strings

bin/Tk/FIJISettingsViewer.pm

@@ -1239,8 +1239,11 @@ sub _rename_fiu_dialog {
     my $rv = $rename_dialog->Show();
 
     my $fiu_name = $e->get();
-    if ($rv eq "OK" && (!$fiu_name =~ /^[\t ]*$/)) {
-        my $name_str = "FIU$i (\"$fiu_name\")";
+    if ($rv eq "OK") {
+        $fiu_name =~ s/^\s+|\s+$//g; # Trim whitespace left and right
+        my $id_str = "FIU$i";
+        my $name_str = $id_str;
+        $name_str .= " (\"$fiu_name\")" if (defined($fiu_name) && length($fiu_name) > 0);
         @{$self->{settings}->{fius}}[$i]->{'FIU_NAME'} = $fiu_name;
         $labframe->configure(-label => $name_str);
         $self->_check_change();