fix: Don't emit empty scip occurrence for builtins

[darichey]

We currently emit occurrences without a symbol name (e.g., { "range": [ 1, 7, 10 ] }) for builtins (e.g., i64). This happens because ide::moniker::def_to_moniker returns None for builtins:

rust-analyzer/crates/ide/src/moniker.rs

Lines 253 to 257 in ca47cdd

	/// * `None` is returned for definitions which are not in a module: `BuiltinAttr`, `BuiltinType`,
	/// `BuiltinLifetime`, `TupleField`, `ToolModule`, and `InlineAsmRegOrRegClass`. TODO: it might be
	/// sensible to provide monikers that refer to some non-existent crate of compiler builtin
	/// definitions.
	pub(crate) fn def_to_moniker(

It never makes sense to emit an occurrence without a symbol name, so just skip in that case.

[Veykril]

We could maybe point the primitive types to their core re-exports https://doc.rust-lang.org/src/core/primitive.rs.html or rustdoc module shims https://doc.rust-lang.org/src/core/primitive_docs.rs.html

[davidbarsky]

I feel like the rustdoc module shims would be more useful for users, but I don't feel too strongly.