Initialize result slices as empty arrays so JSON output is [] not null

[delphinus]

Three handlers declare their result slices with var ... []T, which leaves
them as nil if the loop body never appends. When marshalled to JSON, a nil
Go slice becomes null, not [].

For textDocument/completion, this is a spec violation. CompletionList.items is typed as CompletionItem[], non-nullable. Strict LSP clients can choke on {"isIncomplete": false, "items": null}. I hit this in blink.cmp.

For workspace/symbol and textDocument/documentSymbol, top-level null is technically valid LSP, but [] is the more common convention. The same one-line change keeps the three handlers consistent.

Diff

- var items   []CompletionItem
+ items   := []CompletionItem{}

- var symbols []SymbolInformation   // workspace/symbol
+ symbols := []SymbolInformation{}

- var symbols []SymbolInformation   // documentSymbol
+ symbols := []SymbolInformation{}

No behavior change in Go (len, range, append all behave identically;
no nil comparisons in the codebase). Tiny extra heap allocation per
request (~24 bytes for a slice header). Existing tests pass.