Add UI logic to prevent blank text search in SearchScreen.

- Early return to do not call onSearchExplicitlyTriggered()

Change-Id: I28db284eac9059fb6f48dd718721a1fc5fdb8f7a

feature/search/src/main/kotlin/com/google/samples/apps/nowinandroid/feature/search/SearchScreen.kt

@@ -526,6 +526,7 @@ private fun SearchTextField(
             .focusRequester(focusRequester)
             .onKeyEvent {
                 if (it.key == Key.Enter) {
+                    if (searchQuery.isBlank()) return@onKeyEvent false
                     onSearchExplicitlyTriggered()
                     true
                 } else {
@@ -540,6 +541,7 @@ private fun SearchTextField(
         ),
         keyboardActions = KeyboardActions(
             onSearch = {
+                if (searchQuery.isBlank()) return@KeyboardActions
                 onSearchExplicitlyTriggered()
             },
         ),