[fix] added null check in extension join logic

This avoids a NPE when searching for fields that do not add an alias to
their join (which are all non-extension fields by now, due to fix of
BT-32). This fixes BT-33 (https://jira.osiam.org/browse/BT-33)
Merge pull request #123 from tkrille/BT-33

[fix] added null check in extension join logic