Add null-checks for Map.get()

Add null-checks for Map.get()

Map.get() can return null, and this can mean two different things:

1) The value associated with this key is 'null'.
2) There is no such key in this map.

Java 8 type annotations (Map<K, @NonNull V> for example) can cover
for the first case, but not the second! Map.get() can *always*
return null, no matter the map declaration. The enhanced null analysis
reveals that.

Many places in the code call Map.get() without null-checking, we can
fix these cases in advance. In many cases, this also exposed places
where iterating on map entries should have been done instead of
iterating on keys and doing a lookup.

Change-Id: I8b94a7ed7193f41479b3b0bc9b5990213b9718a3
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/52015
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>