On puppet 3 an explicit undef value is evaluated as the symbol :undef in
ERB templates, while on puppet 4 the explicit undef is evaluated the
same as the implicit undef which comes to the nil value in ERB. Check
for both values to make both puppet versions happy.
Change-Id: Ib4bff7259fea2722f799745d476d1af0c34408d5
4ef66c2bee