Pad with hole == 0 gets not saved correctly

Description

In the Pad-properties dialog it's possible to create a THT-Pad with hole-size==0. (It's questionable if this is useful, but it's possible).

The hole-size of these THT-Pad gets not saved correctly to the board/footprint-file. Instead the (drill xx.xx)-property of this pad is completely omitted at filesave. So at the next loading of a board/footprint these Pad gets a default hole-size of 0.762mm assigned.

example line of below used board-file:

• (pad "2" thru_hole circle (at 0 2.54) (size 2.123 2.123) (layers *.Cu *.Mask) (tstamp 7a75daaa-e1c0-4a26-a723-63d9a2dc84bb))

example project: board_tht-hole_zero_drill.zip

affects: v6.04 + v6.99

Steps to reproduce

1. look at the downloaded board-file with an texteditor, look at line 114: no drill-parameter saved for this Pad2
2. open the project, open Board-file.
3. notice after opening: Pad2 of X2 now contains hole-size==0.762mm
4. doubleclick Pad2 of X2, set hole-size==0, save (see picture 1)
5. close board
6. reopen board
7. notice after reopening: Pad2 of X2 now again contains a hole-size==0.762mm

KiCad Version

Application: KiCad PCB Editor (64-bit)

Version: (6.99.0-1732-g4cd48cd5c9), release build

Libraries:
	wxWidgets 3.1.5
	FreeType 2.11.1
	HarfBuzz 2.8.2
	FontConfig 2.13.1

Platform: Windows 10 (build 19043), 64-bit edition, 64 bit, Little endian, wxMSW

	wxWidgets: 3.1.5 (wchar_t,wx containers)
	Boost: 1.76.0
	OCC: 7.6.0
	Curl: 7.78.0-DEV
	ngspice: 36
	Compiler: Visual C++ 1929 without C++ ABI

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON