Support include empty config file. v5.0.173 v6.0.68 #2626

Job	Run time
multiple-arch-aarch64	11m 44s
cygwin64-cache	5m 14s
multiple-arch-armv7	7m 41s
utest-regression-blackbox-test	6m 15s
cygwin64	9m 13s
fast	1s
build-ubuntu20	2m 26s
build-cross-aarch64	6m 55s
build-cross-arm	9m 48s
build-centos7	4m 1s
build-ubuntu16	2m 41s
build-ubuntu18	2m 19s
coverage	3m 5s
multiple-arch-amd64	2m 5s
test-done	1s
	1h 13m 29s

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