Include NULL in expression builder available functions list

Cherry-picked from 7b18f67

resources/function_help/NULL

@@ -0,0 +1,14 @@
+<h3>NULL value</h3>
+Equates to a null value.
+
+<h4>Syntax</h4>
+<pre>NULL</pre>
+
+<h4>Arguments</h4>
+None
+
+<h4>Example</h4>
+<pre> NULL &rarr; returns a null value </pre>
+
+<h4>Note:</h4>
+<p>To test for NULL use an <pre>IS NULL</pre> or <pre>IS NOT NULL</pre> expression.</p>

src/gui/qgsexpressionbuilderwidget.cpp

@@ -406,6 +406,8 @@ void QgsExpressionBuilderWidget::updateFunctionTree()
   registerItem( "Conditionals", "CASE", casestring );
   registerItem( "Conditionals", "CASE ELSE", caseelsestring );
 
+  registerItem( "Fields and Values", "NULL", "NULL" );
+
   // Load the functions from the QgsExpression class
   int count = QgsExpression::functionCount();
   for ( int i = 0; i < count; i++ )