Embind: Fix uno_Any.get for null interface values

...which had caused an additional getNull/isNull check for
a469aea9c0 "Emscripten: Towards a working C++ UNO
bridge" to fail

Change-Id: Ibe87ca05f795253c9ede8cab6f96da8fe4496f87
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/167344
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>

static/source/unoembindhelpers/PrimaryBindings.cxx

@@ -370,7 +370,10 @@ EMSCRIPTEN_BINDINGS(PrimaryBindings)
                     auto const ifc = *static_cast<css::uno::XInterface* const*>(self.getValue());
                     auto const copy = std::malloc(sizeof(css::uno::XInterface*));
                     *static_cast<css::uno::XInterface**>(copy) = ifc;
-                    ifc->acquire();
+                    if (ifc != nullptr)
+                    {
+                        ifc->acquire();
+                    }
                     emscripten::internal::WireTypePack argv(std::move(copy));
                     return emscripten::val::take_ownership(
                         _emval_take_value(getTypeId(self.getValueType()), argv));