pcbnew: tracks & vias defaults lost at project upgrade from .pro to .kicad_pro

user defined tracks & vias sizes lost during project upgrade to new format

Description

Expected behavior: user defined sizes preserved

Current behavior: Board setup->Defaults->Tracks & Vias are empty after conversion

Steps to reproduce

1. Unzip archive attached kicad_bug.zip (actually any project with .pro file extension)
2. Start KiCad, open pcbnew. Go to Board setup->Defaults->Tracks & Vias: lists are empty
3. Open appeared .kicad_pro file, find line

      "track_widths": [],

4. unzip archive again, open original .kicad_pcb with text editor, find lines

    (user_trace_width 0.3175)
    (user_trace_width 0.5)
    (user_trace_width 1)

I can suppose same behavior with via sizes, but I can't check because I always used default vias only so noone my old pcb has user via sizes defined.

KiCad Version

Application: Pcbnew

Version: (5.99.0-6452-gf8a4edb1c), release build

Libraries:
	wxWidgets 3.0.4

Platform: Linux 5.4.0-52-generic x86_64, 64 bit, Little endian, wxGTK, cinnamon, x11

	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.22
	Boost: 1.65.1
	Curl: 7.58.0
	ngspice: 31
	Compiler: GCC 7.5.0 with C++ ABI 1011

Build settings:
	KICAD_SCRIPTING=OFF
	KICAD_SCRIPTING_MODULES=OFF
	KICAD_SCRIPTING_PYTHON3=OFF
	KICAD_SCRIPTING_WXPYTHON=OFF
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
	KICAD_SCRIPTING_ACTION_MENU=OFF
	BUILD_GITHUB_PLUGIN=OFF
	KICAD_SPICE=ON