Hide Return floating button in Duck.ai when input is empty (#6860)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/488551667048375/task/1211455649223884?focus=true

### Description

- Hides the return button on the Duck.ai tab on the Input Screen if
nothing has been entered.

### Steps to test this PR

- [x] Open the Input Screen and go to the Duck.ai tab
- [x] Verify that the return button is hidden
- [x] Type something
- [x] Verify that the return button is shown

Author: joshliebe

duckchat/duckchat-impl/src/main/java/com/duckduckgo/duckchat/impl/inputscreen/ui/viewmodel/InputScreenViewModel.kt

@@ -363,7 +363,10 @@ class InputScreenViewModel @AssistedInject constructor(
     fun onChatInputTextChanged(query: String) {
         chatInputTextState.value = query.trim()
         _visibilityState.update {
-            it.copy(showChatLogo = (query == initialSearchInputText && !it.autoCompleteSuggestionsVisible) || query.isEmpty())
+            it.copy(
+                showChatLogo = (query == initialSearchInputText && !it.autoCompleteSuggestionsVisible) || query.isEmpty(),
+                newLineButtonVisible = query.isNotBlank(),
+            )
         }
     }