make-binaries: Allow for empty INSTALL_DIR_FOR_OTP

There's probably no actual use case for setting INSTALL_DIR_FOR_OTP to
an empty value, this change is just for consistency with ejabberd's
tools/make-binaries script.

scripts/make-binaries

@@ -277,7 +277,7 @@ add_otp_path()
 	if [ "$mode" = 'native' ]
 	then
 		native_otp_bin="$prefix/bin"
-	elif [ -n "${INSTALL_DIR_FOR_OTP:+x}" ]
+	elif [ -n "${INSTALL_DIR_FOR_OTP+x}" ]
 	then
 		# Let GitHub runners build for non-native systems:
 		# https://github.com/erlef/setup-beam#environment-variables
@@ -563,7 +563,7 @@ else
 	targets="$*"
 fi
 
-if [ "$build_native" = 'false' ] && [ -z "${INSTALL_DIR_FOR_OTP:+x}" ]
+if [ "$build_native" = 'false' ] && [ -z "${INSTALL_DIR_FOR_OTP+x}" ]
 then
 	error 'Native Erlang/OTP installation not found.'
 	error "Set INSTALL_DIR_FOR_OTP or add the '$platform' target."