plot_heatmaps / plot_heatmaps_logarithmic crash on their own default dimensions=None

[ivankrylov]

Both functions declare dimensions=None as the default (src/graphing/plotting.py:633
and :759), but get_heatmap_data immediately does len(dimensions)
(src/graphing/heatmap.py:207):
TypeError: object of type 'NoneType' has no len()
plot_heatmaps_logarithmic fails slightly differently: TypeError: '>' not supported
between instances of 'float' and 'NoneType'.

get_heatmap_data already has a friendly error path printing the expected dimensions
format when the list is the wrong length — None just never reaches it. Either guard
for None there, or make dimensions a required positional argument since there's no
usable default.