[bugfix] When serializing XML, if the prefix points to the same namespace as the default namespace, then remove the qname namespace prefix

Closes eXist-db/exist#5790

Author: adamretter

exist-core/pom.xml

@@ -921,6 +921,7 @@
                                 <include>src/main/java/org/exist/util/serializer/DOMStreamer.java</include>
                                 <include>src/main/java/org/exist/util/serializer/EXISerializer.java</include>
                                 <include>src/test/java/org/exist/util/serializer/HTML5WriterTest.java</include>
+                                <include>src/main/java/org/exist/util/serializer/SAXSerializer.java</include>
                                 <include>src/main/java/org/exist/util/serializer/SerializerObjectFactory.java</include>
                                 <include>src/main/java/org/exist/util/serializer/json/JSONObject.java</include>
                                 <include>src/test/java/org/exist/util/serializer/json/JSONObjectTest.java</include>
@@ -1320,6 +1321,7 @@
                                 <exclude>src/main/java/org/exist/util/serializer/DOMStreamer.java</exclude>
                                 <exclude>src/main/java/org/exist/util/serializer/EXISerializer.java</exclude>
                                 <exclude>src/test/java/org/exist/util/serializer/HTML5WriterTest.java</exclude>
+                                <exclude>src/main/java/org/exist/util/serializer/SAXSerializer.java</exclude>
                                 <exclude>src/main/java/org/exist/util/serializer/SerializerObjectFactory.java</exclude>
                                 <exclude>src/main/java/org/exist/util/serializer/json/JSONObject.java</exclude>
                                 <exclude>src/test/java/org/exist/util/serializer/json/JSONObjectTest.java</exclude>

exist-core/src/main/java/org/exist/util/StringUtil.java

@@ -293,4 +293,24 @@ public static boolean endsWith(@Nullable final String string, @Nullable final St
 
         return string.substring(0, idx);
     }
+
+    /**
+     * Test if two strings are equal whilst handling null references.
+     *
+     * @param s1 the first string.
+     * @param s2 the second string.
+     *
+     * @return true of the strings are equal or both are null, false otherwise.
+     */
+    public static boolean equals(@Nullable final String s1, @Nullable final String s2) {
+        if (s1 == s2) {
+            return true;
+        }
+
+        if (s1 == null) {
+            return false;
+        }
+
+        return s1.equals(s2);
+    }
 }