Fix mypy errors.

Author: a18rhodes

pandas/core/generic.py

@@ -7127,12 +7127,10 @@ def fillna(
                         "All columns must have the same dtype, but got dtypes: "
                         f"{dict(zip(result.columns, dtypes))}"
                     )
-                if isinstance(value, dict):
-                    value = np.asarray(value)
-                if value.dtype != dtypes[0]:
+                if (value_dtype := np.asarray(value).dtype) != dtypes[0]:
                     raise ValueError(
                         "Dtype mismatch for value "
-                        f"(value.dtype={value.dtype} vs {dtypes[0]})"
+                        f"(value.dtype={value_dtype} vs {dtypes[0]})"
                     )
                 for i, _ in enumerate(result):
                     # rowise update by index in case of duplicate column names