Fix test failure with rustc 1.47.0 from Bullseye

On v2021 we updated the rustc compiler to 1.47.0 from Bullseye as it was required elsewhere, but it is causing the librsvg build to fail with:

---- property_bag::tests::property_bag_can_iterate_from_c stdout ----
thread 'property_bag::tests::property_bag_can_iterate_from_c' panicked at 'attempted to leave type `attributes::Attribute` uninitialized, which is invalid', /usr/src/rustc-1.47.0/library/core/src/mem/mod.rs:666:9

Earlier in the log the compiler warns about the relevant code:

warning: the type `attributes::Attribute` does not permit being left uninitialized
   --> rsvg_internals/src/property_bag.rs:276:32
    |
276 |         let mut att = unsafe { mem::uninitialized() };
    |                                ^^^^^^^^^^^^^^^^^^^^
    |                                |
    |                                this code causes undefined behavior when executed
    |                                help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |

Blindly do what the compiler suggest to make the test pass again.

In newer upstream version of librsvg this code has been dropped as part of a larger refactoring.