Fix: List Index out out range with empty parts [] (#138)

* Fix: Index out out range with empty parts []

while question was blocked by api, the parts is an empty list [], 
so the exp: 
if len(parts) > 1 or "text" not in parts[0]:
will raise list index out of range, but not ValueError

* simplify parts length check.

* Update error message.

---------

Co-authored-by: Mark Daoust <[email protected]>

Author: Andy963

google/generativeai/types/generation_types.py

@@ -324,14 +324,13 @@ def text(self):
             ValueError: If the candidate list or parts list does not contain exactly one entry.
         """
         parts = self.parts
-        if len(parts) > 1 or "text" not in parts[0]:
+        if len(parts) != 1 or "text" not in parts[0]:
             raise ValueError(
                 "The `response.text` quick accessor only works for "
-                "simple (single-`Part`) text responses. This response "
-                "contains multiple `Parts`. Use the `result.parts` "
-                "accessor or the full "
+                "simple (single-`Part`) text responses. This response is not simple text."
+                "Use the `result.parts` accessor or the full "
                 "`result.candidates[index].content.parts` lookup "
-                "instead"
+                "instead."
             )
         return parts[0].text