json output: don't set 'unknown' for missing data

Instead of setting 'unknown' or '<unknown>' just pass NULL to json_*
function, which results in omitting the data.

src/output-json-file.c

@@ -93,7 +93,7 @@ static json_t *LogFileMetaGetUri(const Packet *p, const File *ff) {
         }
     }
 
-    return json_string("<unknown>");
+    return NULL;
 }
 
 static json_t *LogFileMetaGetHost(const Packet *p, const File *ff) {
@@ -112,7 +112,7 @@ static json_t *LogFileMetaGetHost(const Packet *p, const File *ff) {
         }
     }
 
-    return json_string("<unknown>");
+    return NULL;
 }
 
 static json_t *LogFileMetaGetReferer(const Packet *p, const File *ff) {
@@ -136,7 +136,7 @@ static json_t *LogFileMetaGetReferer(const Packet *p, const File *ff) {
         }
     }
 
-    return json_string("<unknown>");
+    return NULL;
 }
 
 static json_t *LogFileMetaGetUserAgent(const Packet *p, const File *ff) {
@@ -160,7 +160,7 @@ static json_t *LogFileMetaGetUserAgent(const Packet *p, const File *ff) {
         }
     }
 
-    return json_string("<unknown>");
+    return NULL;
 }
 
 /**
@@ -201,8 +201,6 @@ static void FileWriteJsonRecord(JsonFileLogThread *aft, const Packet *p, const F
         SCFree(s);
     if (ff->magic)
         json_object_set_new(fjs, "magic", json_string((char *)ff->magic));
-    else
-        json_object_set_new(fjs, "magic", json_string("unknown"));
     switch (ff->state) {
         case FILE_STATE_CLOSED:
             json_object_set_new(fjs, "state", json_string("CLOSED"));

src/output-json-http.c

@@ -197,8 +197,6 @@ static void JsonHttpLogJSON(JsonHttpLogThread *aft, json_t *js, htp_tx_t *tx)
             json_object_set_new(hjs, "hostname", json_string(c));
             SCFree(c);
         }
-    } else {
-        json_object_set_new(hjs, "hostname", json_string("<unknown>"));
     }
 
     /* uri */
@@ -222,8 +220,6 @@ static void JsonHttpLogJSON(JsonHttpLogThread *aft, json_t *js, htp_tx_t *tx)
             json_object_set_new(hjs, "http_user_agent", json_string(c));
             SCFree(c);
         }
-    } else {
-        json_object_set_new(hjs, "http_user_agent", json_string("unknown>"));
     }
 
     /* x-forwarded-for */