[FLI-777] Preserve previous snapshot if parser fails

[Mark Phelps]

flipt-client-sdks/flipt-engine/lib/evaluation/src/store/mod.rs

Line 50 in 0432b92

 let doc = parser.parse(n).unwrap_or(Document::default()); 

Instead of blanking out the state with a new Document, we should preserve the last known state.

_FLI-777