fig.add_axes() with no arguments silently does nothing

Bug summary

Running fig.add_axes() without any arguments does nothing (and doesn't raise an error).

Code for reproduction

import matplotlib.pyplot as plt

fig = plt.figure()
new_axes = fig.add_axes()
print(new_axes)

Actual outcome

new_axes is None

the relevant code in add_axes reads:

...
if not len(args):
    return
...

Expected outcome

either of these:

Matplotlib version

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