Add nullptr checks to wxGridTableBase::Set(Row|Col)Attr

wxGridTableBase::SetAttr (for cells) does check its attr parameter for
nullptr, but the check was missing from the row and column functions.
Adding it makes it possible to use a nullptr argument to reset the
attributes.

src/generic/grid.cpp

@@ -1055,6 +1055,7 @@ void wxGridTableBase::SetRowAttr(wxGridCellAttr *attr, int row)
 {
     if ( m_attrProvider )
     {
+        if ( attr )
             attr->SetKind(wxGridCellAttr::Row);
         m_attrProvider->SetRowAttr(attr, row);
     }
@@ -1070,6 +1071,7 @@ void wxGridTableBase::SetColAttr(wxGridCellAttr *attr, int col)
 {
     if ( m_attrProvider )
     {
+        if ( attr )
             attr->SetKind(wxGridCellAttr::Col);
         m_attrProvider->SetColAttr(attr, col);
     }