Allow null string to be customised for QgsDateTimeEdit

E.g. for processing use this needs to be "Not specified"

python/gui/auto_generated/editorwidgets/qgsdatetimeedit.sip.in

@@ -103,6 +103,26 @@ Set the current date as NULL.
 Resets the widget to show no value (ie, an "unknown" state).
 
 .. versionadded:: 2.16
+%End
+
+    QString nullRepresentation() const;
+%Docstring
+Returns the widget's NULL representation, which defaults
+to :py:func:`QgsApplication.nullRepresentation()`
+
+.. seealso:: :py:func:`setNullRepresentation`
+
+.. versionadded:: 3.14
+%End
+
+    void setNullRepresentation( const QString &null );
+%Docstring
+Sets the widget's ``null`` representation, which defaults
+to :py:func:`QgsApplication.nullRepresentation()`
+
+.. seealso:: :py:func:`nullRepresentation`
+
+.. versionadded:: 3.14
 %End
 
   signals:

src/gui/editorwidgets/qgsdatetimeedit.cpp

@@ -36,6 +36,7 @@ QgsDateTimeEdit::QgsDateTimeEdit( QWidget *parent )
 
 QgsDateTimeEdit::QgsDateTimeEdit( const QVariant &var, QVariant::Type parserType, QWidget *parent )
   : QDateTimeEdit( var, parserType, parent )
+  , mNullRepresentation( QgsApplication::nullRepresentation() )
 {
   QIcon clearIcon = QgsApplication::getThemeIcon( "/mIconClearText.svg" );
   mClearAction = new QAction( clearIcon, tr( "clear" ), this );
@@ -151,7 +152,7 @@ void QgsDateTimeEdit::focusOutEvent( QFocusEvent *event )
   if ( mAllowNull && mIsNull && !mCurrentPressEvent )
   {
     QAbstractSpinBox::focusOutEvent( event );
-    if ( lineEdit()->text() != QgsApplication::nullRepresentation() )
+    if ( lineEdit()->text() != mNullRepresentation )
     {
       displayNull();
     }
@@ -191,7 +192,7 @@ void QgsDateTimeEdit::showEvent( QShowEvent *event )
 {
   QDateTimeEdit::showEvent( event );
   if ( mAllowNull && mIsNull &&
-       lineEdit()->text() != QgsApplication::nullRepresentation() )
+       lineEdit()->text() != mNullRepresentation )
   {
     displayNull();
   }
@@ -222,6 +223,20 @@ void QgsDateTimeEdit::changed( const QVariant &dateTime )
   emitValueChanged( dateTime );
 }
 
+QString QgsDateTimeEdit::nullRepresentation() const
+{
+  return mNullRepresentation;
+}
+
+void QgsDateTimeEdit::setNullRepresentation( const QString &nullRepresentation )
+{
+  mNullRepresentation = nullRepresentation;
+  if ( mIsNull )
+  {
+    lineEdit()->setText( mNullRepresentation );
+  }
+}
+
 void QgsDateTimeEdit::displayNull( bool updateCalendar )
 {
   disconnect( this, &QDateTimeEdit::dateTimeChanged, this, &QgsDateTimeEdit::changed );
@@ -232,7 +247,7 @@ void QgsDateTimeEdit::displayNull( bool updateCalendar )
     QDateTimeEdit::setDateTime( minimumDateTime() );
   }
   lineEdit()->setCursorPosition( lineEdit()->text().length() );
-  lineEdit()->setText( QgsApplication::nullRepresentation() );
+  lineEdit()->setText( mNullRepresentation );
   connect( this, &QDateTimeEdit::dateTimeChanged, this, &QgsDateTimeEdit::changed );
 }
 

src/gui/editorwidgets/qgsdatetimeedit.h

@@ -97,6 +97,24 @@ class GUI_EXPORT QgsDateTimeEdit : public QDateTimeEdit
      */
     void setEmpty();
 
+    /**
+     * Returns the widget's NULL representation, which defaults
+     * to QgsApplication::nullRepresentation().
+     *
+     * \see setNullRepresentation()
+     * \since QGIS 3.14
+     */
+    QString nullRepresentation() const;
+
+    /**
+     * Sets the widget's \a null representation, which defaults
+     * to QgsApplication::nullRepresentation().
+     *
+     * \see nullRepresentation()
+     * \since QGIS 3.14
+     */
+    void setNullRepresentation( const QString &null );
+
   signals:
 
     /**
@@ -141,6 +159,7 @@ class GUI_EXPORT QgsDateTimeEdit : public QDateTimeEdit
 
     QString mOriginalStyleSheet = QString();
     QAction *mClearAction;
+    QString mNullRepresentation;
 
     //! TRUE if the widget allows null values
     bool mAllowNull = true;