for tests MOZILLA_CERTIFICATE_FOLDER should be set to empty not 0

0 means it has a value (string "0") but we check for an empty
string. The end result is the same however.

Change-Id: I01fed189eaf37673fdb254884c5e7382914b5211
Reviewed-on: https://gerrit.libreoffice.org/65766
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

solenv/gbuild/gbuild.mk

@@ -245,7 +245,7 @@ gb_GLOBALDEFS := $(sort $(gb_GLOBALDEFS))
 # * Cap the number of threads unittests use:
 gb_TEST_ENV_VARS := MAX_CONCURRENCY=4
 # * Disable searching for certificates by default:
-gb_TEST_ENV_VARS += MOZILLA_CERTIFICATE_FOLDER=0
+gb_TEST_ENV_VARS += MOZILLA_CERTIFICATE_FOLDER=
 # Avoid hanging if the cups daemon requests a password:
 gb_TEST_ENV_VARS += SAL_DISABLE_SYNCHRONOUS_PRINTER_DETECTION=1
 ifeq (,$(SAL_USE_VCLPLUGIN))