Apparent regression in support for empty prefix

[paulmillar]

I'm trying to use robot template to accept a CSV file that has IRIs that take advantage of the empty prefix; e.g., :1234.

bin/robot template --template in.csv --prefix ": http://example.org/foo/" --output out.ttl

With v1.8.1, this worked, but with later versions this fails with the message:

PREFIX LOAD ERROR Could not load prefix '' for ' http://example.org/foo/'

Running git bisect it looks like commit 9cc988a is the first one containing the problem.