Cleanup StringModuleInstanceIdentifierCodec a bit
Compact the implementation a bit by reusing code paths. Also we do not
really allow null defaultPrefix, but check for it being empty.
Make this.defaultPrefix nullable, which allows us to simplify the check
in moduleForPrefix() down to a simple null check.
JIRA: NETCONF-804
Change-Id: Ifa01017613761466a32e371d56b0bb6521145115
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
63eba7f355bacfe2aa1f20112cf2a7f98bd2928c)