[Bug Report] Rendering broken in v0.25

[vmoens]

Describe the bug
Rendering is broken for simple envs (e.g. Pendulum-v1)

Code example

import gym
from gym.wrappers.pixel_observation import PixelObservationWrapper
PixelObservationWrapper(gym.make("Pendulum-v1")).reset()

Error stack:

/path/to/venv/python3.8/site-packages/gym/wrappers/pixel_observation.py in __init__(self, env, pixels_only, render_kwargs, pixel_keys)
    131             pixels: np.ndarray = pixels[-1] if isinstance(pixels, List) else pixels
    132 
--> 133             if np.issubdtype(pixels.dtype, np.integer):
    134                 low, high = (0, 255)
    135             elif np.issubdtype(pixels.dtype, np.float):

AttributeError: 'NoneType' object has no attribute 'dtype'

System Info
Describe the characteristic of your environment:

• Describe how Gym was installed (pip, docker, source, ...)

pip install gym --upgrade

• What OS/version of Linux you're using. Note that while we will accept PRs to improve Window's support, we do not officially support it.
  OsX (x86) and Unix
• Python version
  3.7, 3.8, 3.9 and 3.10

Checklist

• I have checked that there is no similar issue in the repo (required)

EDIT

I guess this is related to this (from the release note)

Render - The render API is changed such that the mode has to be specified during gym.make with the keyword render_mode, after which, the render mode is fixed. For further details see https://younis.dev/blog/2022/render-api/ and #2671. This has the additional changes
with render_mode="human" you don't need to call .render(), rendering will happen automatically on env.step()
with render_mode="rgb_array", .render() pops the list of frames rendered since the last .reset()
with render_mode="single_rgb_array", .render() returns a single frame, like before.

Specifying render_mode="single_rgb_array" solves it, but it is a bc-breaking change. Any chance we can get a default value there?