Protect against empty strings resolving to badly defined units

Apparently the German translation of unit definitions yields empty match strings for two units, which necessitated commit 6e6c5707 in kdeplasma-addons (for the converter runner accepting "1/2" as a number with valid unit).

This fixes the problem at the root, by preventing units added to m_unitMap with empty match strings as keys. I also include a warning message so these problems hopefully get fixed in the relevant languages as well.