[xbase][trace] Don’t skip empty root regions

We need to store the path information for the tracing to
work properly.

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=448020

Change-Id: I79185f5281269d67e13a581c158b5401944402d7

Conflicts:
	plugins/org.eclipse.xtext.xbase/src/org/eclipse/xtext/xbase/compiler/output/TreeAppendable.java
[xbase][validation] Avoid NPE since it’ll break the final field init check

Change-Id: I8e8a1db287f138bb9d7acc6927bae5d89985108c
[xtend][contentAssist] Avoid exception from illegal scope query

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=450827

Change-Id: I2435f5a4a8b04b7abf6581897e51cb2532f6e9fe
[xtend][contentAssist] Fixed exception on CA

Also removed unnecessarily duplicated code.

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=450911

Change-Id: Ia15598ad78de93b90e91576e69606dde602a1632