BUG: pandas.testing.assert_frame_equal with check_dtype=False unexpectedly raises an exception
This code

```
import pandas
from datetime import datetime
df1 = pandas.DataFrame({'a': []}) 
df2 = pandas.DataFrame({'a': []})
df1['a'] = pandas.to_datetime(df1['a'])
pandas.testing.assert_frame_equal(df1, df2, check_dtype=False)
```

raises: 
```
AssertionError: right is not an ExtensionArray

```

But should it raise instead no exception as df1 and df2 are identical, except for their dtypes ? 
