Minor fix: Parse the body to JSON only if it's defined
Minor fix: Parse the body to JSON only if it's defined

When the application/json header is received from a client, the
request's body is parsed to JSON. However, this should be done only if
the body contains a string to be parsed.

inverno/restbase,wikimedia/hyperswitch,d00rman/restbase,milimetric/restbase,Krenair/restbase,cscott/restbase,inverno/restbase,cscott/restbase,milimetric/restbase,physikerwelt/restbase,wikimedia/restbase,eevans/restbase,wikimedia/mediawiki-services-restbase,gwicke/restbase,gwicke/restbase,wikimedia/mediawiki-services-restbase,Pchelolo/restbase,d00rman/restbase,eevans/restbase,Krenair/restbase,physikerwelt/restbase,wikimedia/restbase,Pchelolo/restbase