Detect empty apex in ConfiguredJarList

Previously, ConfiguredJarList would accept an empty apex name,
e.g. ":jar" which makes no sense as every apex has to have a non-empty
name. This change makes an empty apex invalid.

In order to improve the test coverage of the TestConfiguredJarList test
this change also changes the implementation of
CreateTestConfiguredJarList([]string) to marshal the supplied strings
into a json list and then unmarshal into a ConfiguredJarList which more
closely matches how it is used at runtime.

Bug: 178361284
Test: m nothing
Change-Id: I7dfec6b4cc1923aa99746e976da0393922ef0791