Fix port-part matches algorithm for a null port input (#435)

The algorithm was expecting a non-null possibly-empty string while being called with null or a non-empty string. Now the algorithm correctly handles a null or non-empty input string.

index.bs

@@ -4153,16 +4153,19 @@ Content-Type: application/reports+json
     `port-part` matching
   </h5>
 
-  An <a>ASCII string</a> |input| <dfn export>`port-part` matches</dfn> [=/URL=] |url| if a CSP source expression that
-  contained the first as a <a grammar>`port-part`</a> could potentially match a URL containing the latter's [=url/port=]
-  and [=url/scheme=]. For example, "80" <a>`port-part` matches</a> matches http://example.com.
+  An <a>ASCII string</a> or null |input| <dfn export>`port-part` matches</dfn>
+  [=/URL=] |url| if a CSP source expression that contained the first as a <a
+  grammar>`port-part`</a> could potentially match a URL containing the latter's
+  [=url/port=] and [=url/scheme=]. For example, "80" <a>`port-part` matches</a>
+  matches http://example.com.
 
   <ol class="algorithm">
-    1.  Assert: |input| is the empty string, "*", or a sequence of <a>ASCII digits</a>.
+    1.  Assert: |input| is null, "*", or a sequence of one or more <a>ASCII digits</a>.
 
     2.  If |input| is equal to "*", return "`Matches`".
 
-    3.  Let |normalizedInput| be null if |input| is the empty string; otherwise |input| interpreted as decimal number.
+    3.  Let |normalizedInput| be null if |input| null; otherwise |input|
+        interpreted as decimal number.
 
     4.  If |normalizedInput| equals |url|'s [=url/port=], return "`Matches`".