Empty system variables for file paths in @Sources cause URISyntaxException failures

[jonn-smith]

When using a variable in the @Sources annotation for a file path, if that variable is empty / undefined a URISyntaxException is thrown.

This is because of the resulting call to the URI constructor:

new URI("file:")

However, this produces an empty, non-null URI and is equivalent:

new URI("")

The logic in ConfigURIFactory::newURI should be updated to allow for this case.

==========

My example is below.

If BasicTestConfigWithClassPathOverridesAndVariableFile.pathToGatkConfig is not defined as a system variable, then at ConfigURIFactory.java:44 a URISyntaxException is thrown.

/**
 * Simple test configuration file to check overrides and other configuration features.
 */
@Config.LoadPolicy(Config.LoadType.MERGE)
@Config.Sources({
        "file:${" + BasicTestConfigWithClassPathOverridesAndVariableFile.CONFIG_FILE_VARIABLE_FILE_NAME + "}",
        "file:/etc/jaiow",                          // Test of non-existent bad path
        "classpath:org/broadinstitute/hellbender/utils/config/BasicTestConfigWithClassPathOverrides.properties",
})
public interface BasicTestConfigWithClassPathOverridesAndVariableFile extends Mutable, Accessible {
    String CONFIG_FILE_VARIABLE_FILE_NAME = "BasicTestConfigWithClassPathOverridesAndVariableFile.pathToGatkConfig";

    @DefaultValue("true")
    boolean booleanDefTrue();
}