Most/all C++ based IO code had a pattern of doing using
RNA_struct_property_is_set to check whether a default path needs to
be set. However, it returns false for properties restored from
"previous operator settings" (property restoration code sets
IDP_FLAG_GHOST flag on them, which "is set" sees and goes
"nope, not set").
The fix here is to apply similar logic as 10 years ago in the
T32855 fix (rBdb250a4): use RNA_struct_property_is_set_ex instead.
Reviewed By: Campbell Barton
Differential Revision: https://developer.blender.org/D15904
97bd04d665