load-fragment: reset the list on an ExecStart= containing only whitespace

This is consistent with how an empty string works in an ExecStart=
statement.  We should not differentiate between an empty string and
whitespace only (since they look the same.)

Update the test case with whitespace only to reflect that the list is
reset.

Tested that `test-unit-file` passes and other test cases are not
affected.  Installed the patched systemd binaries on a machine, booted
it, looked for out of the usual behavior but did not find any.

src/test/test-unit-file.c

@@ -145,19 +145,19 @@ static void test_config_parse_exec(void) {
         assert_se(r == 0);
         assert_se(c1->command_next == NULL);
 
-        log_info("/* no command, check for bad memory access */");
+        log_info("/* no command, whitespace only, reset */");
         r = config_parse_exec(NULL, "fake", 3, "section", 1,
                               "LValue", 0, "    ",
                               &c, NULL);
         assert_se(r == 0);
-        assert_se(c1->command_next == NULL);
+        assert_se(c == NULL);
 
         log_info("/* ignore && honour_argv0 */");
         r = config_parse_exec(NULL, "fake", 4, "section", 1,
                               "LValue", 0, "-@/RValue///slashes3 argv0a r1",
                               &c, NULL);
         assert_se(r >= 0);
-        c1 = c1->command_next;
+        c1 = c;
         check_execcommand(c1, "/RValue/slashes3", "argv0a", "r1", NULL, true);
 
         log_info("/* ignore && honour_argv0 */");