Default is a valid netclass, even if it's not in m_NetClasses.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/17388

(cherry picked from commit dac16a02cc)

eeschema/erc.cpp

@@ -559,6 +559,7 @@ int ERC_TESTER::TestMissingNetclasses()
 {
     int                            err_count = 0;
     std::shared_ptr<NET_SETTINGS>& settings = m_schematic->Prj().GetProjectFile().NetSettings();
+    wxString                       defaultNetclass = settings->m_DefaultNetClass->GetName();
 
     auto logError =
             [&]( const SCH_SHEET_PATH& sheet, SCH_ITEM* item, const wxString& netclass )
@@ -590,8 +591,11 @@ int ERC_TESTER::TestMissingNetclasses()
                             {
                                 wxString netclass = field->GetText();
 
-                                if( settings->m_NetClasses.count( netclass ) == 0 )
+                                if( !netclass.IsSameAs( defaultNetclass )
+                                        && settings->m_NetClasses.count( netclass ) == 0 )
+                                {
                                     logError( sheet, item, netclass );
+                                }
                             }
                         }