metomi/rose#376: fix trigger for null-value missing parent

This addresses #376.

Previously, a missing parent was only handled for some-values triggers like

<samp>trigger=env=A: 20, 30</samp>

but not

<samp>trigger=env=A</samp>

(it skipped to the <samp>else: # Enabled parent, value is ok.</samp> block).