Metamodel: ignore missing optional modules for ceylon/ceylon-compiler#1523
FreeModule: avoid NPE for default and Java modules (who do not have a module_.class)
Metamodel: return null for non-available modules (for ceylon/ceylon-compiler#1523)
Metamodel: null-checking module util method used by the JVM compiler for ceylon/ceylon-compiler#1523
Tests for ceylon/ceylon-compiler#1523