fix(button-group): margins in Safari

Closes #7858

Author: MPopov

projects/igniteui-angular/src/lib/core/styles/components/button-group/_button-group-theme.scss

@@ -204,6 +204,10 @@
         position: relative;
         z-index: 0;
         transition: $transition;
+        // The margin here is required to fix a bug in Safari #7858
+        margin-top: 0;
+        margin-#{$right}: 0;
+        margin-bottom: 0;
 
         &%igx-button--flat {
             border-radius: 0;