[runners/converter] Skip empty parts when splitting units

Summary:
Skip empty parts when splitting the units({D27166}).
PS: I had to set the locale to german to reproduce this issue,
thats why I didn't noticed it in the first place.

Test Plan: Does not display results for queries like 1/2

Reviewers: broulik

Reviewed By: broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28458