Schematic Parity check complains about duplicate "not in schematic" refs

Description

The Schematic Parity test complains when footprints that are set "Not in schematic" have the same reference. Since they're not-in-schematic they shouldn't be subject to parity test at all. Usually, when I have not-in-schematic footprints they are logos/graphics, so it never occurs to me to change the reference from the default REF**.

Steps to reproduce

1. Place two footprints directly into the PCB without changing their default reference
2. Set "Not in schematic" on both
3. Run DRC with parity check enabled

KiCad Version

Application: KiCad PCB Editor

Version: 5.99.0-unknown-11749+g6427197962+bullseye, release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.74.0 OpenSSL/1.1.1k zlib/1.2.11 brotli/1.0.9 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh2/1.9.0 nghttp2/1.43.0 librtmp/2.3

Platform: Linux 5.10.0-8-amd64 x86_64, 64 bit, Little endian, wxGTK, xfce, x11

Build Info:
	Date: Aug 18 2021 07:01:49
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.74.0
	OCC: 7.5.1
	Curl: 7.74.0
	ngspice: 34
	Compiler: GCC 10.2.1 with C++ ABI 1014

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON