out_dxf: fix empty $DIMBLK #4705

Job	Run time
linux (CFLAGS="-O1 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindin...	6m 56s
linux (CC="clang")	6m 52s
linux (--enable-release)	32m 59s
linux (--enable-gcov --disable-bindings --disable-docs CFLAGS="-O0 -g")	5m 33s
linux (--disable-write --disable-docs)	6m 41s
linux (CFLAGS="-O2 -m32" --disable-bindings --disable-docs)	8m 6s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	10m 8s
linux-cmake	2m 59s
macOS	7m 11s
mingw	30m 21s
mips32-big-endian	9m 43s
ppc64-big-endian	8m 1s
mingw-cmake	3m 19s
vs2019	4m 58s
vs2019-ninja	4m 28s
	2h 28m 15s

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