http.server: handle an empty Json body

[wilzbach]

If req.json is called on an empty Json body, it throws an error message which
is rather hard to understand for the casual observer:

core.exception.AssertError@/usr/include/dlang/dmd/std/typecons.d(2583): Called `get' on null Nullable!Json.
----------------
??:? _d_assert_msg [0xb539aeea]
??:? inout pure nothrow ref @property @nogc @safe inout(vibe.data.json.Json) std.typecons.Nullable!(vibe.data.json.Json).Nullable.get() [0xb5224d99]
??:? ref @property @safe vibe.data.json.Json vibe.http.server.HTTPServerRequest.json() [0xb521431d]
??:? void app.handler(scope vibe.http.server.HTTPServerRequest, vibe.http.server.HTTPServerResponse) [0xb51e4b4e]
??:? @trusted void vibe.http.server.listenHTTP!(vibe.http.server.HTTPServerSettings).listenHTTP(vibe.http.server.HTTPServerSettings, void function(scope vibe.http.server.HTTPServerRequest, vibe.http.server.HTTPServerResponse)*).__lambda3!(vibe.http.server.HTTPServerRequest, vibe.http.server.HTTPServerResponse).__lambda3(scope vibe.http.server.HTTPServerRequest, scope vibe.http.server.HTTPServerResponse) [0xb51eea3b]
??:? @safe bool vibe.http.server.handleRequest(vibe.internal.interfaceproxy.InterfaceProxy!(vibe.core.stream.Stream).InterfaceProxy, vibe.core.net.TCPConnection, vibe.http.server.HTTPServerContext, ref vibe.http.server.HTTPServerSettings, ref bool, scope stdx.allocator.IAllocator) [0xb5286a8d]
??:? @trusted void vibe.http.server.handleHTTPConnection(vibe.core.net.TCPConnection, vibe.http.server.HTTPServerContext).__lambda4() [0xb5284f46]
??:? @safe void vibe.http.server.handleHTTPConnection(vibe.core.net.TCPConnection, vibe.http.server.HTTPServerContext) [0xb5284c57]
??:? nothrow @safe void vibe.http.server.listenHTTPPlain(vibe.http.server.HTTPServerSettings, void delegate(vibe.http.server.HTTPServerRequest, vibe.http.server.HTTPServerResponse) @safe).doListen(vibe.http.server.HTTPServerContext, bool, bool).__lambda4(vibe.core.net.TCPConnection) [0xb52185a5]
??:? void vibe.core.task.TaskFuncInfo.set!(void delegate(vibe.core.net.TCPConnection) @safe, vibe.core.net.TCPConnection).set(ref void delegate(vibe.core.net.TCPConnection) @safe, ref vibe.core.net.TCPConnection).callDelegate(ref vibe.core.task.TaskFuncInfo) [0xb5351580]
??:? void vibe.core.task.TaskFuncInfo.call() [0xb533184d]
??:? nothrow void vibe.core.task.TaskFiber.run() [0xb5330e65]
??:? void core.thread.Fiber.run() [0xb53e7edb]
??:? fiber_entryPoint [0xb53e7daa]
??:? [0xffffffff]

[PetarKirov]

remove

[PetarKirov]

void main()

[PetarKirov]

And perhaps runEventLoop() before the end of main?

[wilzbach]

(remove VibeDefaultMain)