Fix for Set-AzSqlManagedInstance to not set Geo be default (#12921)

Co-authored-by: Perica Trajkov <[email protected]>

src/Sql/Sql/ManagedInstance/Services/AzureSqlManagedInstanceAdapter.cs

@@ -261,7 +261,7 @@ public static string MapExternalBackupStorageRedundancyToInternal(string backupS
                 case "Zone":
                     return "ZRS";
                 default:
-                    return "GRS";
+                    return null;
             }
         }
 
@@ -281,7 +281,7 @@ public static string MapInternalBackupStorageRedundancyToExternal(string backupS
                 case "ZRS":
                     return "Zone";
                 default:
-                    return "Geo";
+                    return null;
             }
         }
     }