Have SemanticTokensFull return nil when no symbols are available for the file

[doriable]

When a file does not have parsed symbols, we are currently
sending a nil slice for the data in semantic tokens.

The specification for textDocument/semanticTokens/full outlines that
the expected response is SemanticTokens | null, where SemanticTokens
is defined as:

export interface [SemanticTokens](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#semanticTokens) {
	/**
	 * An optional result id. If provided and clients support delta updating
	 * the client will include the result id in the next semantic token request.
	 * A server can then instead of computing all semantic tokens again simply
	 * send a delta.
	 */
	resultId?: string;

	/**
	 * The actual tokens.
	 */
	data: [uinteger](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#uinteger)[];
}

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_semanticTokens

data is not an optional field for SemanticTokens, so setting it to nil
causes failures on clients:

From the LSP logs:

DEBUG\tresponding\t{"method": "textDocument/semanticTokens/full", "params": {"data":null}}\n'

We should instead be returning nil for SemanticTokensFull in this case, which
is expected by clients.

LSP logs after the change:

DEBUG\tresponding\t{"method": "textDocument/semanticTokens/full", "params": null}\n

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Dec 12, 2024, 9:52 PM

[bufdev]

Can you point to how this makes a difference? Downstream code should generally not care whether this is 0 or nil. If there's a specific reason, this should be documented here with a code comment so that we understand this choice in the future.

[doriable]

Updated the PR, title, and description: I did some digging into the specification, and textDocument/semanticTokens/full expects SemanticTokens | null as the response, but data on SemanticTokens is not optional. So in this case, if we are not sending any data, we should instead be returning nil for semantic tokens. I added this explanation as in-line comments in the code change as well.