Handle optionals correctly

Currently, "unfilled" optionals (i.e., ones that do not have a child), lead to a NullPointerException because the getX method is called without testing for their existence first.

MWE:

Root ::= [A] ;
A ;

Root root = new Root();  // do not set A
Dumper.read(root).dumpAsPNG(somePath);

Will lead to

Caused by: java.lang.NullPointerException
	at ast.ASTNode.getChild(ASTNode.java:...)
	at ast.Root.getA(Root.java:...)