Fix ExpectationMessage.update(failureMessage:) not to update a FailureMessage instance with empty string

The followings is the previous behavior:

- `FailureMessage().toExpectationMessage()` will be `ExpectationMessage.fail("")`
- `ExpectationMessage.fail("").update(failureMessage: aMessage)` will set `aMessage.stringValue = ""`
- Thus the error message of `expect(1).to(MatcherFunc { _, _ in false }.predicate)` will be empty

Author: ikesyo

Sources/Nimble/ExpectationMessage.swift

@@ -152,8 +152,10 @@ public indirect enum ExpectationMessage {
     // Backwards compatibility: converts ExpectationMessage tree to FailureMessage
     internal func update(failureMessage: FailureMessage) {
         switch self {
-        case let .fail(msg):
+        case let .fail(msg) where !msg.isEmpty:
             failureMessage.stringValue = msg
+        case .fail:
+            break
         case let .expectedTo(msg):
             failureMessage.actualValue = nil
             failureMessage.postfixMessage = msg