MINOR: sample: improve error reporting on missing arg to strcmp() converter

Calling the strcmp() converter with no argument yields this strange error:

  [ALERT]    (31439) : parsing [test.cfg:3] : error detected in frontend 'f' while parsing 'http-request redirect' rule : failed to parse sample expression <src,strcmp]> : invalid args in converter 'strcmp' : failed to register variable name ''.

This is because the vars name check tries to see if it can create such a
variable having an empty name. Let's at least make a special case of the
missing argument. Now we can read a more explicit:

  [ALERT]    (31655) : parsing [test.cfg:3] : error detected in frontend 'f' while parsing 'http-request redirect' rule : failed to parse sample expression <src,strcmp]> : invalid args in converter 'strcmp' : missing variable name.

This was done for secure_strcmp() as well.

src/sample.c

@@ -3591,6 +3591,11 @@ static int sample_conv_mqtt_is_valid(const struct arg *arg_p, struct sample *smp
 static int smp_check_strcmp(struct arg *args, struct sample_conv *conv,
                            const char *file, int line, char **err)
 {
+	if (!args[0].data.str.data) {
+		memprintf(err, "missing variable name");
+		return 0;
+	}
+
 	/* Try to decode a variable. */
 	if (vars_check_arg(&args[0], NULL))
 		return 1;
@@ -3607,6 +3612,11 @@ static int smp_check_strcmp(struct arg *args, struct sample_conv *conv,
 static int smp_check_secure_memcmp(struct arg *args, struct sample_conv *conv,
                            const char *file, int line, char **err)
 {
+	if (!args[0].data.str.data) {
+		memprintf(err, "missing variable name");
+		return 0;
+	}
+
 	/* Try to decode a variable. */
 	if (vars_check_arg(&args[0], NULL))
 		return 1;