Problem with backward compatibility: if not <string> raises exception

Steps to reproduce:
1. Create application with following class:
Namespaces:
  =: io.murano.apps
  std: io.murano
  sys: io.murano.system

Name: HelloReporter
Extends: std:Application
Properties:
  name:
    Contract: $.string().notNull()
Workflow:
  initialize:
    Body:
      - $.environment: $.find(std:Environment).require()
  deploy:
    Body:
      - If: not $.getAttr(deployed, false)
        Then:
          - $.environment.reporter.report($this, 'Starting deployment! Hello!')
          - $.setAttr(deployed, True)

2. Create environment and add this application to it
3. Deploy it
4. After deploy add one more app to the env
5. Deploy it again

Observed result:
Deployment failure:
ERROR murano.common.engine [-]
  yaql.language.exceptions.NoMatchingFunctionException: No function "#unary_operator_not" matches supplied arguments
  Traceback (most recent call last):
    File "/tmp/murano-packages-cache/7f060584ee32479d81e3b267ff4ad793/a8a4390870d147349a467e9f277b0094/Classes/Environment.yaml", line 70:9 in method deploy of class io.murano.Environment
        $.applications.pselect($.deploy())
    File "/tmp/murano-packages-cache/7f060584ee32479d81e3b267ff4ad793/439241e09e6049c589c5de5c9f1b72b0/Classes/HelloReporter.yaml", line 22:13 in method deploy of class io.murano.apps.HelloReporter
        not $.getAttr(deployed, false)
    File "/opt/stack/murano/murano/dsl/macros.py", line 121 in method execute
        res = self._condition(context)
    File "/opt/stack/murano/murano/dsl/yaql_expression.py", line 82 in method __call__
        return self._parsed_expression.evaluate(context=context)
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/expressions.py", line 165 in method evaluate
        return self(utils.NO_VALUE, context, self.engine)
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/expressions.py", line 156 in method __call__
        return super(Statement, self).__call__(receiver, context, engine)
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/expressions.py", line 37 in method __call__
        return context(self.name, engine, receiver, context)(*self.args)
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/contexts.py", line 65 in method <lambda>
        data_context, use_convention, function_filter)
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/runner.py", line 49 in method call
        name, all_overloads, engine, receiver, data_context, args, kwargs)
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/runner.py", line 117 in method choose_overload
        args = tuple(arg_evaluator(i, arg) for i, arg in enumerate(args))
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/runner.py", line 117 in method <genexpr>
        args = tuple(arg_evaluator(i, arg) for i, arg in enumerate(args))
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/runner.py", line 113 in method <lambda>
        and not isinstance(arg, expressions.Constant))
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/expressions.py", line 37 in method __call__
        return context(self.name, engine, receiver, context)(*self.args)
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/contexts.py", line 65 in method <lambda>
        data_context, use_convention, function_filter)
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/runner.py", line 49 in method call
        name, all_overloads, engine, receiver, data_context, args, kwargs)
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/runner.py", line 141 in method choose_overload
        raise_not_found()
    File "/usr/local/lib/python2.7/dist-packages/yaql/language/runner.py", line 70 in method raise_not_found
        raise exceptions.NoMatchingFunctionException(name)