Not connected pins should not get connected

This prevents splitting wires/busses on the NC pin end.  It also keeps
NC pins out of the netlist.

Fixes https://gitlab.com/kicad/code/kicad/issues/6777

eeschema/connection_graph.cpp

@@ -27,6 +27,7 @@
 #include <profile.h>
 #include <common.h>
 #include <erc.h>
+#include <pin_type.h>
 #include <sch_bus_entry.h>
 #include <sch_component.h>
 #include <sch_edit_frame.h>
@@ -526,6 +527,10 @@ void CONNECTION_GRAPH::updateItemConnectivity( const SCH_SHEET_PATH& aSheet,
 
             for( SCH_PIN* pin : component->GetPins( &aSheet ) )
             {
+                // Not connected pins are not connected.
+                if( pin->GetType() == ELECTRICAL_PINTYPE::PT_NC )
+                    continue;
+
                 pin->InitializeConnection( aSheet, this );
 
                 wxPoint pos = pin->GetPosition();

eeschema/sch_component.cpp

@@ -1796,6 +1796,9 @@ bool SCH_COMPONENT::doIsConnected( const wxPoint& aPosition ) const
 
     for( const auto& pin : m_pins )
     {
+        if( pin->GetType() == ELECTRICAL_PINTYPE::PT_NC )
+            continue;
+
         // Collect only pins attached to the current unit and convert.
         // others are not associated to this component instance
         int pin_unit = pin->GetLibPin()->GetUnit();