[pax-url-aether] Do not check invalid PID.settings locations

[grgrzybek]

For now it's:

org.ops4j.pax.url/pax-url-aether/src/main/java/org/ops4j/pax/url/mvn/internal/config/MavenConfigurationImpl.java

Lines 196 to 199 in b993f16

	fallbacks.add(m_propertyResolver.get(key));
	fallbacks.add(System.getProperty("user.home") + "/.m2/settings.xml");
	fallbacks.add(System.getProperty("maven.home") + "/conf/settings.xml");
	fallbacks.add(System.getenv("M2_HOME") + "/conf/settings.xml");

But some are obviously not available when given props/env variables are null.