fix(--var function): update the -var function to use 'null' as a default value for fallback param. (#5600)

Author: desig9stein

projects/igniteui-angular/src/lib/core/styles/base/utilities/_functions.scss

@@ -349,12 +349,15 @@
 ///   // }
 ///   // otherwise, it will return the value for key 'icon-color' in the '$avatar-theme';
 /// @returns {String} - The value of the key in the passed map, or the name of key concatenated with the key name.
-@function --var($map, $key, $fallback: '') {
+@function --var($map, $key, $fallback: null) {
     $igx-legacy-support: if(global-variable-exists('igx-legacy-support'), $igx-legacy-support, true);
 
     @if map-has-key($map, $key) {
         @if $igx-legacy-support == false {
-            @return unquote('var(--#{map-get($map, 'name')}-#{$key}, #{$fallback})');
+            @if not($fallback) {
+                @return unquote('var(--#{map-get($map, 'name')}-#{$key})')
+            }
+            @return unquote('var(--#{map-get($map, 'name')}-#{$key}, #{$fallback})')
         } @else {
             @return map-get($map, $key);
         }