check for empty geometry when plotting

[akaszynski]

Encountered a fairly alarming and cryptic error when running:

from ansys.mapdl.core import launch_mapdl
mapdl = launch_mapdl()
mapdl.aplot()

Error traceback included being unable to update an empty geometry cache.

Area plotting now includes a basic test to verify that when there is no geometry to plot, nothing is plotted (as in standard MAPDL behavior).

[codecov]

Codecov Report

Merging #999 (86bc06c) into main (bd80c49) will increase coverage by 1.46%.
The diff coverage is 96.30%.

@@            Coverage Diff             @@
##             main     #999      +/-   ##
==========================================
+ Coverage   73.28%   74.74%   +1.46%     
==========================================
  Files          39       43       +4     
  Lines        5750     6043     +293     
==========================================
+ Hits         4214     4517     +303     
+ Misses       1536     1526      -10