Common type of two strings where only one has values is too restricted

The TypeCalculator.common_type(t1,t2) method, when given two Strings where only one of the Strings has values will yield a common String type that is restricted to those values. This is wrong. If one of the types is unrestricted, then the common type must also be unrestricted.

QA

risk: low
probability: low
severity: low
test level: unit