Make RuleBuilder fail fast when passed a nil Path

Previously, the RuleBuilder would add all supplied Paths directly to
one of its lists of Paths without checking to make sure it was not nil
only to panic somewhere in its Build() method when it attempted to
convert it to a string. Deferring the failure made it harder to debug.
This change fails fast by checking every Path when it is passed to the
RuleBuilder.

Test: m nothing
Change-Id: I06b35da02101f6dfab80d2daaf3d8e44ded8b6be