async: client: fix uninitialized proxy error on clang

When compiling with Clang, the following error is raised:
```
../include/sdbusplus/async/client.hpp:36:39: error: field 'proxy' is uninitialized when used here [-Werror,-Wuninitialized]
        : Types<decltype(proxy)>(ctx, proxy)..., ctx(ctx)
```

Fix this by passing a temporarily initialized proxy, which would have
the same value as the one to-be-constructed.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I2ab8ad05102799b94a25a823e1dbcfc7e1b70053
diff --git a/include/sdbusplus/async/client.hpp b/include/sdbusplus/async/client.hpp
index ce9fc24..4bece52 100644
--- a/include/sdbusplus/async/client.hpp
+++ b/include/sdbusplus/async/client.hpp
@@ -33,7 +33,7 @@
     /* Default (empty) constructor only when Service and Path are missing. */
     explicit client(sdbusplus::async::context& ctx)
         requires(!S && !P)
-        : Types<decltype(proxy)>(ctx, proxy)..., ctx(ctx)
+        : Types<decltype(proxy)>(ctx, decltype(proxy){})..., ctx(ctx)
     {}
 
     /* Conversion constructor for a non-empty (Service and/or Path) proxy. */