Relax validator to support [^] any attribute prefix

This discovered functionality previously worked.

Relates to #2079

src/main/java/org/jsoup/select/Evaluator.java

@@ -191,7 +191,7 @@ public static final class AttributeStarting extends Evaluator {
         private final String keyPrefix;
 
         public AttributeStarting(String keyPrefix) {
-            Validate.notEmpty(keyPrefix);
+            Validate.notNull(keyPrefix); // OK to be empty - will find elements with any attributes
             this.keyPrefix = lowerCase(keyPrefix);
         }
 

src/test/java/org/jsoup/select/SelectorTest.java

@@ -1230,4 +1230,17 @@ public void wildcardNamespaceMatchesNoNamespace() {
         Elements els = doc.select(q);
         assertEquals(3, els.size());
     }
+
+    @Test public void emptyAttributePrefix() {
+        // https://github.com/jhy/jsoup/issues/2079
+        // Discovered feature: [^] should find elements with any attribute (any prefix)
+        String html = "<p one>One<p one two>Two<p>Three";
+        Document doc = Jsoup.parse(html);
+
+        Elements els = doc.select("[^]");
+        assertSelectedOwnText(els, "One", "Two");
+
+        Elements emptyAttr = doc.select("p:not([^])");
+        assertSelectedOwnText(emptyAttr, "Three");
+    }
 }