RPC - accounts_pending return invalid json when account does not have pending transactions

[rotilho]

When using account_pending action the node returns invalid json (empty string instead of empty map)

Request: {"accounts":["xrb_3534c73cym6pbb9nr6f1waqc3ta98o1nfbupy4w74h9xzp5zi3mbturb3ujn"],"threshold":"1","count":"9223372036854775807","action":"accounts_pending"}

Response: {"blocks":{"xrb_3534c73cym6pbb9nr6f1waqc3ta98o1nfbupy4w74h9xzp5zi3mbturb3ujn":""}}

Correct response {"blocks":{"xrb_3534c73cym6pbb9nr6f1waqc3ta98o1nfbupy4w74h9xzp5zi3mbturb3ujn":"{}"}}

[PlasmaPower]

Duplicate of #499. This is a problem with the boost JSON library we're currently using.