Add null variables for nav-link

Replaces #30498 by adding four new null default variables for .nav-link. Doesn't carry over font-style from the original PR though since that's rarely used, at least by default Bootstrap. Nullifies all values from that PR, too, since we count on some basic inheritance here and don't need color by default.

scss/_nav.scss

@@ -14,11 +14,15 @@
 .nav-link {
   display: block;
   padding: $nav-link-padding-y $nav-link-padding-x;
+  @include font-size($nav-link-font-size);
+  font-weight: $nav-link-font-weight;
+  color: $nav-link-color;
   text-decoration: if($link-decoration == none, null, none);
   @include transition($nav-link-transition);
 
   &:hover,
   &:focus {
+    color: $nav-link-hover-color;
     text-decoration: if($link-hover-decoration == underline, none, null);
   }
 

scss/_variables.scss

@@ -843,6 +843,10 @@ $zindex-tooltip:                    1070 !default;
 
 $nav-link-padding-y:                .5rem !default;
 $nav-link-padding-x:                1rem !default;
+$nav-link-font-size:                null !default;
+$nav-link-font-weight:              null !default;
+$nav-link-color:                    null !default;
+$nav-link-hover-color:              null !default;
 $nav-link-transition:               color .15s ease-in-out, background-color .15s ease-in-out, border-color .15s ease-in-out !default;
 $nav-link-disabled-color:           $gray-600 !default;