Fixed segfaults on Manjaro

On Manjaro Linux, shutter would segfault whenever it would receive
a message from another running instance of itself. This was caused
by the fact that dbus did not handle correctly messages with empty
text. This patch adds an extra check to prevent that.

bin/shutter

@@ -442,12 +442,14 @@ else {
                     #set export filename
                 }
                 elsif ( $cmd eq 'exfilename' ) {
-                    $sc->set_export_filename( $message->get_text );
+                    my $arg = defined $message->get ? $message->get_text : "";
+                    $sc->set_export_filename( $arg );
 
                     #set delay
                 }
                 elsif ( $cmd eq 'delay' ) {
-                    $sc->set_delay( $message->get_text );
+                    my $arg = defined $message->get ? $message->get_text : "";
+                    $sc->set_delay( $arg );
 
                     #set include_cursor flag
                 }
@@ -464,12 +466,13 @@ else {
                     #set nosession flag
                 }
                 elsif ( $cmd eq 'nosession' ) {
-                    $sc->set_no_session( $message->get_text );
+                    my $arg = defined $message->get ? $message->get_text : "";
+                    $sc->set_no_session( $arg );
 
                     #open new files
                 }
                 elsif ( $cmd eq 'fopen' ) {
-                    my @init_files = $message->get_uris;
+                    my @init_files = defined $message->get ? $message->get_uris : ();
                     if ( scalar @init_files > 0 ) {
                         &fct_open_files(@init_files);
                     }
@@ -483,7 +486,8 @@ else {
                     #take screenshot
                 }
                 else {
-                    &evt_take_screenshot( 'global_keybinding', $cmd, undef, $message->get_text );
+                    my $arg = defined $message->get ? $message->get_text : "";
+                    &evt_take_screenshot( 'global_keybinding', $cmd, undef, $arg );
                 }
             }