out_dxf: fix empty $DIMBLK #4708

Job	Run time
macOS	7m 42s
linux (CFLAGS="-O1 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindin...	5m 28s
linux-cmake	1m 59s
linux (CC="clang")	10m 0s
linux (--enable-release)	33m 40s
mingw	30m 41s
linux (--enable-gcov --disable-bindings --disable-docs CFLAGS="-O0 -g")	5m 35s
mingw-cmake	3m 21s
linux (--disable-write --disable-docs)	5m 13s
mips32-big-endian	9m 20s
linux (CFLAGS="-O2 -m32" --disable-bindings --disable-docs)	4m 1s
ppc64-big-endian	10m 2s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	8m 51s
vs2019	5m 16s
vs2019-ninja	5m 10s
	2h 26m 19s

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