element.__init_default: treat `None` plugin_conf as if missing file + refactor

This change is motivated by a simple refactor, there isn't a bug affected by it.

Change to still initialize Element.__defaults as usual, even if a plugin_conf is not specified.

As part of the work on !1101 (merged) to introduce ArtifactElements, a special case for __init_defaults was made, such that none of the usual defaults would be loaded. This seems to be because it has no config and none of the defaults are required by ArtifactElement, although they don't seem to adversely affect it.

Instead, treat a 'None' plugin_conf in the same way as the existing case of a missing file. This avoids the appearance of having plugin-specific behaviour in the base-class, and is perhaps less puzzling to new contributors.