Allow null algorithm for pools.

src/crypto/common/Algorithm.h

@@ -30,6 +30,9 @@
 #include <vector>
 
 
+#include "rapidjson/fwd.h"
+
+
 namespace xmrig {
 
 
@@ -91,6 +94,8 @@ public:
     inline bool operator==(const Algorithm &other) const  { return isEqual(other); }
     inline operator Algorithm::Id() const                 { return m_id; }
 
+    rapidjson::Value toJSON() const;
+
     static Family family(Id id);
     static Id parse(const char *name);