Allow 0 length JSON requests. 0 len XML and multipart already allowed.

Author: allanrbo

CHANGES

@@ -1,6 +1,8 @@
 DD MMM YYYY - 2.9.3 - To be released
 ------------------------------------
 
+ * Allow 0 length JSON requests.
+   [Issue #1822 - @allanbomsft, @zimmerle, @victorhora, @marcstern]
  * Include unanmed JSON values in unnamed ARGS
    [Issue #1577, #1576 - @marcstern, @victorhora, @zimmerle]
  * Fix buffer size for utf8toUnicode transformation 

apache2/msc_reqbody.c

@@ -710,7 +710,7 @@ apr_status_t modsecurity_request_body_end(modsec_rec *msr, char **error_msg) {
         }
         else if (strcmp(msr->msc_reqbody_processor, "JSON") == 0) {
 #ifdef WITH_YAJL
-            if (json_complete(msr, &my_error_msg) < 0) {
+            if (json_complete(msr, &my_error_msg) < 0 && msr->msc_reqbody_length > 0) {
                 *error_msg = apr_psprintf(msr->mp, "JSON parser error: %s", my_error_msg);
                 msr->msc_reqbody_error = 1;
                 msr->msc_reqbody_error_msg = *error_msg;