Push rule evaluation tweaks (#2897)

This tweaks push rule evaluation:

1. to be more strict around pattern matching and to not match empty
patterns
3. to bail if we come across a `dont_notify`, since cycles after that
are wasted
4. refactors `ActionsToTweaks` to make a bit more sense

internal/pushrules/evaluate.go

@@ -145,6 +145,11 @@ func conditionMatches(cond *Condition, event *gomatrixserverlib.Event, ec Evalua
 }
 
 func patternMatches(key, pattern string, event *gomatrixserverlib.Event) (bool, error) {
+	// It doesn't make sense for an empty pattern to match anything.
+	if pattern == "" {
+		return false, nil
+	}
+
 	re, err := globToRegexp(pattern)
 	if err != nil {
 		return false, err
@@ -154,12 +159,20 @@ func patternMatches(key, pattern string, event *gomatrixserverlib.Event) (bool,
 	if err = json.Unmarshal(event.JSON(), &eventMap); err != nil {
 		return false, fmt.Errorf("parsing event: %w", err)
 	}
+	// From the spec:
+	// "If the property specified by key is completely absent from
+	// the event, or does not have a string value, then the condition
+	// will not match, even if pattern is *."
 	v, err := lookupMapPath(strings.Split(key, "."), eventMap)
 	if err != nil {
 		// An unknown path is a benign error that shouldn't stop rule
 		// processing. It's just a non-match.
 		return false, nil
 	}
+	if _, ok := v.(string); !ok {
+		// A non-string never matches.
+		return false, nil
+	}
 
 	return re.MatchString(fmt.Sprint(v)), nil
 }