Undefined parameters (null) are undefined

e.g.

pkgs/petsc.py

    if stage_args['coptflags']:
        conf_lines.append('COPTFLAGS=%s' % stage_args['coptflags'])

If coptflags is not defined in the profile, the condition evaluates to
False.
Merge pull request #202 from hashdist/undefined_is_empty

Undefined parameters (null) are undefined