hooks: matplotlib: assert that mpl_data_dir is not empty

If the statement in exec_statement() fails for whatever reason,
mpl_data_dir ends up empty, which in turn cases the current working
directory being collected as matplotlib/mpl-data.

This leads to difficult-to-debug issues (see confusion around .git
directory under #5504), so it's better to just signal the error.

PyInstaller/hooks/hook-matplotlib.py

@@ -14,6 +14,7 @@
 
 mpl_data_dir = exec_statement(
     "import matplotlib; print(matplotlib.get_data_path())")
+assert mpl_data_dir, "Failed to determine matplotlib's data directory!"
 
 datas = [
     (mpl_data_dir, "matplotlib/mpl-data"),