indxf: allow empty num_classes for <=r2000 #4700

Job	Run time
linux-cmake	2m 56s
linux (CFLAGS="-O1 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindin...	7m 47s
macOS	7m 25s
linux (CC="clang")	6m 46s
mips32-big-endian	8m 52s
ppc64-big-endian	7m 56s
linux (--enable-release)	33m 5s
linux (--enable-gcov --disable-bindings --disable-docs CFLAGS="-O0 -g")	5m 24s
mingw	29m 35s
mingw-cmake	3m 36s
linux (--disable-write --disable-docs)	6m 56s
vs2019	6m 10s
linux (CFLAGS="-O2 -m32" --disable-bindings --disable-docs)	7m 34s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	10m 46s
vs2019-ninja	4m 43s
	2h 29m 31s

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