[clangd] Treat "null" optional fields as missing

Clangd currently throws away any protocol messages whenever an optional
field has an unexpected type. This patch changes the behaviour to treat
`null` fields as missing.

This enables clangd to be more tolerant against small violations to the
LSP spec.

Fixes https://github.com/clangd/vscode-clangd/issues/134

Differential Revision: https://reviews.llvm.org/D95229