refactor: remove ts 3.7 compatibility type cast in ripple-rend… (#17934)

8fd9d7d added a compatibility type cast
to the ripple renderer. To actually fix the assignment in a non-cast
way that works for TS 3.6 and TS 3.7, we just assign the
`backgroundColor` style property if actually needed.

Author: devversion

src/material/core/ripple/ripple-renderer.ts

@@ -146,9 +146,12 @@ export class RippleRenderer {
     ripple.style.height = `${radius * 2}px`;
     ripple.style.width = `${radius * 2}px`;
 
-    // If the color is not set, the default CSS color will be used.
-    // TODO(TS3.7): Type 'string | null' is not assignable to type 'string'.
-    ripple.style.backgroundColor = (config.color || null) as any;
+    // If a custom color has been specified, set it as inline style. If no color is
+    // set, the default color will be applied through the ripple theme styles.
+    if (config.color != null) {
+      ripple.style.backgroundColor = config.color;
+    }
+
     ripple.style.transitionDuration = `${duration}ms`;
 
     this._containerElement.appendChild(ripple);