fix(text-area): counter supports null value (#2089)

Author: bartosjiri

src/TextArea/TextArea.svelte

@@ -85,7 +85,7 @@
       </label>
       {#if maxCount}
         <div class:bx--label={true} class:bx--label--disabled={disabled}>
-          {value.length}/{maxCount}
+          {(value ?? "").length}/{maxCount}
         </div>
       {/if}
     </div>