Avoid a panic in QuoteSQLIdent for empty input

[apstndb]

Problem

token.QuoteSQLIdent("") panics with an index-out-of-range error because needQuoteSQLIdent reads s[0] without first checking whether s is empty.

panic: runtime error: index out of range [0] with length 0

The same panic is reachable indirectly through an exported AST node:

_ = (&ast.Ident{}).SQL()

Reproduction

token.QuoteSQLIdent("")

https://go.dev/play/p/vniPESiuWGD

package main

import "github.com/cloudspannerecosystem/memefish/token"

func main() {
	_ = token.QuoteSQLIdent("")
}

(*ast.Ident).SQL()

https://go.dev/play/p/Jp6w9qpqLfE

package main

import "github.com/cloudspannerecosystem/memefish/ast"

func main() {
	_ = (&ast.Ident{Name: ""}).SQL()
}

Why change this

The function returns only a string and documents no non-empty precondition. Downstream code that constructs ast.Ident values from runtime strings must currently validate every identifier before serialization or recover at a broader boundary.

This follows the separation in google/googlesql: ParseIdentifier rejects an empty backquoted identifier, while ToIdentifierLiteral("") returns two backticks and explicitly notes that the result is not a valid identifier.

Expected behavior

QuoteSQLIdent("") returns two backticks instead of panicking. Identifier validation remains unchanged; empty identifiers are still rejected by the lexer/parser.