Fix up 'String dtype: fix convert_dtypes() to convert NaN-string to NA-string'

Author: jorisvandenbossche

pandas/core/internals/blocks.py

@@ -663,7 +663,7 @@ def convert(
             or isinstance(res_values, NumpyExtensionArray)
             and res_values._ndarray is values
         ):
-            res_values = values.copy()
+            res_values = res_values.copy()
         elif res_values is values:
             refs = self.refs