Improve TextBlock code & test

src/Typography/TextBlock.php

@@ -23,6 +23,12 @@ class TextBlock extends Collection
         return $this->items;
     }
 
+    /**
+     * Get line by given key
+     *
+     * @param mixed $key
+     * @return null|Line
+     */
     public function line($key): ?Line
     {
         if (!array_key_exists($key, $this->lines())) {

tests/Typography/TextBlockTest.php

@@ -35,6 +35,7 @@ class TextBlockTest extends TestCase
         $this->assertEquals('foo', $block->line(0));
         $this->assertEquals('FooBar', $block->line(1));
         $this->assertEquals('bar', $block->line(2));
+        $this->assertNull($block->line(20));
     }
 
     public function testLongestLine(): void