fix(reporting): replace mutable default argument in plot_corner (B006)

quantiles=[0.16, 0.5, 0.84] as a default argument is shared across
all calls. If the list were ever mutated (e.g. by a caller or by
corner.corner internally in a future version), all subsequent calls
using the default would receive the corrupted value.

Fix: use None as the sentinel and initialise inside the function body.
The behaviour is identical when callers omit the argument.