Default content to an empty arrayref rather than undef

If $content is undef because Elements had no value, the
JSON::to_json comparison with $new_content can fail with:

    hash- or arrayref expected (not a simple scalar, use allow_nonref to allow this)

We have seen this error when the JSON module is running with
JSON::PP rather than JSON::XS.

share/html/Dashboards/Queries.html

@@ -80,7 +80,7 @@
 } else {
     $title = loc("Modify the content of dashboard [_1]", $Dashboard->Name);
 }
-my $content = $Dashboard->{Attribute}->SubValue('Elements');
+my $content = $Dashboard->{Attribute}->SubValue('Elements') || [];
 
 if ( $ARGS{AddRow} || $ARGS{Update} ) {