text: Title: Remove `if not matches:` from StringEvaluator
  Body: ## Refactor
  
  ### Description
  Remove unuseful lines:
  
  - https://github.com/scanapi/scanapi/blob/main/scanapi/evaluators/string_evaluator.py#L27-L28
  - https://github.com/scanapi/scanapi/blob/main/scanapi/evaluators/string_evaluator.py#L27-L28
  
  `matches` variable is an `callable_iterator object`, so it will never be null. We can remove the check, since when there is no match with the pattern, it will not go into the loop:
  
  ```python
  -> if not matches:
  (Pdb) ll
   23  	    @classmethod
   24  	    def _evaluate_env_var(cls, sequence):
   25  	        matches = cls.variable_pattern.finditer(sequence)
   26  	        import pdb
   27
   28  	        pdb.set_trace()
   29  ->	        if not matches:
   30  	            return sequence
   31
   32  	        for match in matches:
   33  	            variable_name = match.group("variable")
   34
   35  	            if any(letter.islower() for letter in variable_name):
   36  	                continue
   37
   38  	            try:
   39  	                variable_value = os.environ[variable_name]
   40  	            except KeyError as e:
   41  	                raise BadConfigurationError(e)
   42
   43  	            sequence = cls.replace_var_with_value(
   44  	                sequence, match.group(), variable_value
   45  	            )
   46
   47  	        return sequence
  (Pdb) sequence
  'no env var'
  (Pdb) matches
  <callable_iterator object at 0x1112855b0>
  ```
  
  Removing the two lines should also increase the coverage.