Figure.set_panel: Fix the bug when panel is not set

[seisman]

In the GMT CLI, using gmt subplot set without specifying the panel activates the next panel. For example:

gmt begin map
gmt subplot begin 1x2 -Fs6c/3c
gmt subplot set
gmt basemap -R0/3/0/3 -B
gmt subplot set gmt basemap -R0/3/0/3 -B
gmt subplot end
gmt end show

The equivalent PyGMT script is:

importpygmtfig=pygmt.Figure()
withfig.subplot(nrows=1, ncols=2, figsize=("6c", "3c")):
withfig.set_panel():
fig.basemap(region=[0, 3, 0, 3], frame=True)
withfig.set_panel():
fig.basemap(region=[0, 3, 0, 3], frame=True)
fig.show()

But it doesn't work as expected:

subplot [ERROR]: No subplot information file!
...

The bug is because we always pass panel to gmt subplot set even if it's None. The bug can be traced back to the initial implementation in PR #822.

This PR fixes the long-term bug and add a test for it.

[weiji14]

Somewhat prefer having the panel be explicit, but our typehint does say panel: int | Sequence[int] | None = None, so I guess the default of panel=None should work somehow like in GMT.

[seisman]

Sometimes, it's easier to activate the next panel without specifying its index by using panel=True in plotting methods. However, this method can't override the subplot tag. In such cases, we can use Figure.set_panel(tag="XXX"). This is how I discovered the bug.