Value: init "as" member to avoid -Wmaybe-uninitialized warning

[tbleher]

GCC (at least version 13.3) warns when moving a freshly constructed value, since the "as" member is not initialized:

  In constructor 'jsonrpc::Value::Value(jsonrpc::Value&&)',
      inlined from 'jsonrpc::Response::Response(int32_t, std::string, jsonrpc::Value)' at jsonrpc-lean/response.h:27:13,
      inlined from 'std::shared_ptr<jsonrpc::FormattedData> jsonrpc::Server::HandleRequest(const std::string&, const std::string&)' at jsonrpc-lean/server.h:74:17:
  jsonrpc-lean/value.h:124:63: error: '<unnamed>.jsonrpc::Value::as' may be used uninitialized [-Werror=maybe-uninitialized]
    124 |         Value(Value&& other) noexcept : myType(other.myType), as(other.as) {
        |                                                               ^~~~~~~~~~~~
  jsonrpc-lean/server.h: In member function 'std::shared_ptr<jsonrpc::FormattedData> jsonrpc::Server::HandleRequest(const std::string&, const std::string&)':
  jsonrpc-lean/server.h:74:62: note: '<anonymous>' declared here
     74 |                 Response(ex.GetCode(), ex.GetString(), Value()).Write(*writer);
        |

Fix this by initializing the member in the default constructor.

[tbleher]

@uskr I would appreciate if you could have a look at this patch :)