add missing null display driver class for thinknode m7 ethernet variant

variants/thinknode_m7/platformio.ini

@@ -169,6 +169,7 @@ build_src_filter = ${ThinkNode_M7.build_src_filter}
   ${ThinkNode_M7_ethernet.build_src_filter}
   +<helpers/esp32/*.cpp>
   +<helpers/ui/MomentaryButton.cpp>
+  +<helpers/ui/NullDisplayDriver.cpp>
   +<../examples/companion_radio/*.cpp>
   +<../examples/companion_radio/ui-orig/*.cpp>
 lib_deps = ${ThinkNode_M7.lib_deps}