[folks] Use return_val_if_fail() to fix a build error

[Niels De Graef <nielsdg src gnome org>]

commit 343460faa12b46316132e5a24dcf25d90674bffa
Author: Jasper Lievisse Adriaanse <j jasper la>
Date:   Sat May 15 11:39:39 2021 +0200

    Use return_val_if_fail() to fix a build error
    
    error: non-void function '_edsf_persona_get_string_property' should return a value
    
    closes #129

 backends/eds/lib/edsf-persona.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/backends/eds/lib/edsf-persona.vala b/backends/eds/lib/edsf-persona.vala
index 5f03d248..ee7bec6e 100644
--- a/backends/eds/lib/edsf-persona.vala
+++ b/backends/eds/lib/edsf-persona.vala
@@ -2268,7 +2268,7 @@ public class Edsf.Persona : Folks.Persona,
   private unowned string? _get_string_property (string prop_name)
     {
       var field = E.Contact.field_id (prop_name);
-      return_if_fail (E.Contact.field_is_string (field));
+      return_val_if_fail (E.Contact.field_is_string (field), null);
       return contact.get_const<string> (field);
     }