refactor: Update StrInput class to include default value

Update the StrInput class in inputs.py to include a default value of an empty string. This change ensures consistency and improves the usability of the class.

src/backend/base/langflow/inputs/inputs.py

@@ -25,6 +25,7 @@ class StrInput(BaseInputMixin, ListableInputMixin, DatabaseLoadMixin):  # noqa:
     field_type: Optional[SerializableFieldTypes] = FieldTypes.TEXT
     load_from_db: StrictBoolean = False
     """Defines if the field will allow the user to open a text editor. Default is False."""
+    value: Optional[str] = ""
 
 
 class MultilineInput(BaseInputMixin):