code: public String resolveString(String variable, int lineNumber, int startPosition) {
      return Objects.toString(resolveObject(variable, lineNumber, startPosition), "");
    }
docs: Resolve a variable into a string value. If given a string literal (e.g. 'foo' or "foo"), this method returns the literal unquoted. If the variable is undefined in the context, this method returns the given variable string.
  
  @param variable
  name of variable in context
  @param lineNumber
  current line number, for error reporting
  @param startPosition
  current line position, for error reporting
  @return resolved value for variable