[BUGFIX] Exclude absent line number from exception message

The bug was introduced by #1288, so has not been included in any release;
thus a changelog entry is not justified.

Author: JakeQZ

tests/Unit/Parsing/SourceExceptionTest.php

@@ -20,7 +20,7 @@ public function getMessageReturnsMessageProvidedToConstructor(): void
         $message = 'The cake is a lie.';
         $exception = new SourceException($message);
 
-        self::assertStringContainsString($message, $exception->getMessage());
+        self::assertSame($message, $exception->getMessage());
     }
 
     /**
@@ -55,6 +55,17 @@ public function getMessageWithLineNumberProvidedIncludesLineNumber(): void
         self::assertStringContainsString(' [line no: ' . $lineNumber . ']', $exception->getMessage());
     }
 
+    /**
+     * @test
+     */
+    public function getMessageWithLineNumberProvidedIncludesMessage(): void
+    {
+        $message = 'There is no flatware.';
+        $exception = new SourceException($message, 17);
+
+        self::assertStringContainsString($message, $exception->getMessage());
+    }
+
     /**
      * @test
      */