LibWeb: Don't drop single <br/> lines

Previously, when having inline contexts consisting of just a `<br/>`
tag, we would not create a line box.

Ensure that there is always a line box when a line is explicitly being
broken and also ensure it won't be trimmed due to being empty.

This will a fix a number of sites that use `<br>` tags for layouts
between block elements (even though the spec says they shouldn't).

Tests/LibWeb/Layout/input/single-br-inline-layout.html

@@ -0,0 +1,15 @@
+<style>
+    body {
+        font-family: 'SerenitySans';
+    }
+
+    #begin {
+        height: 2px;
+        background-color: blue;
+    }
+
+    #end {
+        height: 2px;
+        background-color: red;
+    }
+</style><div id="begin"></div><br><div id="end"></div>