Fix if condition with no refinement

[CatarinaGamboa]

Before:
When the guard condition had no refinement, we used the default of true.
That, however, became a problem after joining ifs (with no else), since we assumed the condition was true.

Now:
We still create new fresh boolean but leave it empty, leaving the verification to the SMT solver.
We also remove the branches that cannot complete normally from the If joining (return, throw, break, continue).
(This can still be improved if Latte was used.)

Example:

  String loadFirstSetting(String path, String fallback) throws IOException {                                                         
      BufferedReader reader = new BufferedReader(new FileReader(path));                                                              
      String header = reader.readLine();        // reader: open                                                                      
                                                                                                                                     
      if (header.startsWith("#")) {             // condition has no refinement                                                       
          String value = reader.readLine();     // reader: still open                                                                
          reader.close();                       // reader: closed                                                                    
          return value;                                                                                                              
      }
      // Before: State Refinement Error here — the join assumed the                                                         
      //         then-branch was taken, so reader was seen as `closed` and the                                                       
      //         close() below failed its `!closed` precondition.                                                                    
      // Now:    the then-branch returns and contributes nothing to the join,                                                        
      //         so reader is still `open` and the close() is accepted.                                                                                                                 
      reader.close();                           // reader: closed
      return header;
  }