Don't force users to make a choice in the datum transform dialog

It's possible to get a situation where none of the options can be
selected, because they all require a grid which isn't available on
the system. So always allow users to close the dialog to dismiss it
(which has the same effect as picking the default selection from
the dialog when possible, or giving null transforms if NO transforms
are available at all...)

Fixes qgis#34234

(cherry picked from commit 644a552)

src/app/qgsdatumtransformtablewidget.cpp

@@ -291,7 +291,7 @@ void QgsDatumTransformTableWidget::editDatumTransform()
     }
 
 #if PROJ_VERSION_MAJOR>=6
-    if ( sourceCrs.isValid() && destinationCrs.isValid() && !proj.isEmpty() )
+    if ( sourceCrs.isValid() && destinationCrs.isValid() )
 #else
     if ( sourceCrs.isValid() && destinationCrs.isValid() &&
          ( sourceTransform != -1 || destinationTransform != -1 ) )

src/gui/qgsdatumtransformdialog.cpp

@@ -44,7 +44,7 @@ bool QgsDatumTransformDialog::run( const QgsCoordinateReferenceSystem &sourceCrs
     return true;
   }
 
-  QgsDatumTransformDialog dlg( sourceCrs, destinationCrs, false, true, true, qMakePair( -1, -1 ), parent, nullptr, QString(), mapCanvas );
+  QgsDatumTransformDialog dlg( sourceCrs, destinationCrs, false, true, false, qMakePair( -1, -1 ), parent, nullptr, QString(), mapCanvas );
   if ( !windowTitle.isEmpty() )
     dlg.setWindowTitle( windowTitle );
 

src/gui/qgsdatumtransformdialog.h

@@ -76,7 +76,7 @@ class GUI_EXPORT QgsDatumTransformDialog : public QDialog, private Ui::QgsDatumT
                      QgsMapCanvas *mapCanvas = nullptr,
                      const QString &windowTitle = QString() );
 
-    // TODO QGIS 4.0 - remove selectedDatumTransform
+    // TODO QGIS 4.0 - remove selectedDatumTransform, forceChoice
 
     /**
      * Constructor for QgsDatumTransformDialog.