Fix feedback returned when get_match_feedback doesn't find a match

[zeezooz]

Also fix suggestions for the regex case.

Now it works like the original.

[ltOgt]

Without this the default feedback keeps getting modified:

static Feedback default_feedback = Feedback() // <---------- NOTE: NOT FINAL
    ..warning = ''
    ..suggestions = [
      "Use a few words, avoid common phrases"
          "No need for symbols, digits, or uppercase letters"
    ];

static Feedback get_match_feedback(PasswordMatch match, bool is_sole_match) {
     // other cases ...
     // default case:
    return default_feedback;

final extra_feedback = 'Add another word or two. Uncommon words are better.';
    if (feedback != null) {
      feedback.suggestions!.insert(0, extra_feedback);  // <--------- keeps adding this with every run
      if (feedback.warning == null) {
        feedback.warning = '';
      }