Fix final nits

packages/webview_flutter/webview_flutter_wkwebview/example/lib/web_view.dart

@@ -315,6 +315,7 @@ class WebViewController {
   Future<void> loadFile(
     String absoluteFilePath,
   ) {
+    assert(absoluteFilePath.isNotEmpty);
     return _webViewPlatformController.loadFile(absoluteFilePath);
   }
 
@@ -326,6 +327,7 @@ class WebViewController {
     String html, {
     String? baseUrl,
   }) {
+    assert(html.isNotEmpty);
     return _webViewPlatformController.loadHtmlString(
       html,
       baseUrl: baseUrl,

packages/webview_flutter/webview_flutter_wkwebview/ios/Classes/FlutterWebView.m

@@ -615,14 +615,14 @@ - (void)updateUserAgent:(NSString*)userAgent {
 }
 
 /**
- * Validates if the given `argument` is a non-null non-empty string.
+ * Validates if the given `argument` is a non-null, non-empty string.
  *
  * @param argument The argument that should be validated.
  * @param errorDetails An optional NSString variable which will contain a detailed error message in
  * case the supplied argument is not valid.
- * @return `true` if the given `argument` is a valid non-null, non-empty string; otherwise `false`.
+ * @return `YES` if the given `argument` is a valid non-null, non-empty string; otherwise `NO`.
  */
-+ (bool)isValidStringArgument:(id)argument withErrorMessage:(NSString**)errorDetails {
++ (BOOL)isValidStringArgument:(id)argument withErrorMessage:(NSString**)errorDetails {
   if (!argument) {
     if (errorDetails) {
       *errorDetails = @"Argument is nil.";