made unknown @extend optional

i cant find the definition for `mddtp-picker__selected`anywhere. so i made it optional for now so the webpack build works again

src/scss/component.scss

@@ -204,7 +204,7 @@
           pointer-events: all;
           cursor: pointer;
           &:hover {
-            @extend .mddtp-picker__selected;
+            @extend .mddtp-picker__selected !optional;
           }
         }
       }