eo: Don't resolve function calls for NULL objects

eo: Don't resolve function calls for NULL objects

eo_function(NULL) always leads to a no-operation. A this
point it is basically considered the normal operation
with EO to just silently ignore NULL errors.

If the API function "eo_function" belongs to a class that
has not been loaded yet (eg. no object of this type has
been created yet), then the op associated to the API func
may not be defined yet.

Fixes T5715