Treat a missing email subject as empty

src/Util/Email.php

@@ -50,7 +50,7 @@ public function getRecipients(): array
 
     public function getSubject(): string
     {
-        return $this->subject;
+        return $this->subject ?? '';
     }
 
     public function getSource(): string

tests/unit/Module/MailCatcherTest.php

@@ -95,6 +95,24 @@ public function testDontSeeInLastEmailSubject()
         $mailcatcher->dontSeeInLastEmailSubject('Hello world');
     }
 
+    public function testDontSeeInLastEmailSubjectWithoutSubject()
+    {
+        $mailcatcher = new MailCatcherTest_TestClass();
+        $mailcatcher->setLastMessage(new Email(1, [], null, ''));
+
+        $mailcatcher->dontSeeInLastEmailSubject('Hello world');
+    }
+
+    public function testSeeInLastEmailSubjectWithoutSubjectFails()
+    {
+        $mailcatcher = new MailCatcherTest_TestClass();
+        $mailcatcher->setLastMessage(new Email(1, [], null, ''));
+
+        $this->expectException(AssertionFailedError::class);
+
+        $mailcatcher->seeInLastEmailSubject('Hello world');
+    }
+
     public function testLastMessageToNoMessages()
     {
         $handler = new MockHandler([

tests/unit/Util/EmailTest.php

@@ -14,6 +14,18 @@ public function testGetters()
         $this->assertEquals('Source body', $email->getSource());
     }
 
+    public function testMissingSubjectIsEmpty()
+    {
+        $email = Email::createFromMailcatcherData([
+            'id' => 1,
+            'recipients' => ['some@email.com'],
+            'subject' => null,
+            'source' => 'Source body'
+        ]);
+
+        $this->assertSame('', $email->getSubject());
+    }
+
     public function testCreateFromMailcatcherData()
     {
         $email = Email::createFromMailcatcherData([