fix(core): fail-safe check of nullable value (#1031)

Make sure the lookupStyle method in StyleUtils return a string. On old
browsers getPropertyValue() of getComputedStyle() returns a null instead
of empty string.

Fixes #958

src/lib/core/style-utils/style-utils.ts

@@ -98,7 +98,7 @@ export class StyleUtils {
 
     // Note: 'inline' is the default of all elements, unless UA stylesheet overrides;
     //       in which case getComputedStyle() should determine a valid value.
-    return value.trim();
+    return value ? value.trim() : '';
   }
 
   /**