Returning null for unresolved variables

Returning null for unresolved variables

When Blueprints Processor was not able to evaluate a variable, it would
set its value to null.

The expected behaviour would be to set the value to the default repres-
entation in the formal notation as defined by Apache Velocity, which is
a dollar followed by the name of the variable between curly braces. For
example, if the value of the variable pnf-id could not be evaluated in
runtime, its value would be defined as the string "${pnf-id}".

The problem happened during evaluation of the variables that would be
later sent to the template-meshing code for processing.

The fix was to add a check before the value was assigned to the varia-
ble; if the was not null, the assignment will happen normally. Otherwi-
se, if the evaluation resolves to null, the variable receives the defa-
ult value (string "${<variable name>}").

Besides the tests that were put in place to test the code changed for
this fix, two tests were added to the existing test case of the templa-
te meshing code, to act as regression test.

Change-Id: I635afb1eba4c0d45b821811f0119fa6c87ea9542
Issue-ID: CCSDK-1358
Signed-off-by: ottero <rodrigo.ottero@est.tech>