Support include empty config file. v5.0.173 v6.0.68 #2631

Job	Run time
utest-regression-blackbox-test	6m 25s
cygwin64-cache	4m 44s
multiple-arch-armv7	9m 35s
multiple-arch-aarch64	8m 1s
cygwin64	10m 35s
fast	1s
build-ubuntu16	2m 47s
build-ubuntu18	2m 45s
build-cross-aarch64	9m 14s
build-centos7	3m 59s
build-cross-arm	8m 25s
build-ubuntu20	2m 56s
coverage	4m 27s
multiple-arch-amd64	2m 9s
test-done	1s
	1h 16m 4s

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