submodule-config: use explicit empty string instead of strbuf in config_from()

submodule-config: use explicit empty string instead of strbuf in config_from()

Use a string constant instead of an empty strbuf to shorten the code
and make it easier to read.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>