fix: do not expand empty list or associative array under key operators

[spokodev]

Problem

An empty list or empty associative array expanded under a key operator (?, &, ;) emits a spurious name= pair:

parseTemplate('{?list}').expand({ list: [] });   // "?list="   expected ""
parseTemplate('{&list}').expand({ list: [] });   // "&list="   expected ""
parseTemplate('{;list}').expand({ list: [] });   // ";list="   expected ""
parseTemplate('{?o}').expand({ o: {} });         // "?o="      expected ""

RFC 6570 §2.3: "A variable defined as a list value is considered undefined if the list contains zero members" (likewise an associative array with zero members). An undefined value contributes nothing to an expansion, so the result must be the empty string.

An empty string is different — it is defined — and must still expand to name= per §3.2.8. That case is unaffected.

Cause

In getValues, the non-key branch already guarded with tmp.length !== 0, but the key-operator branch pushed key= unconditionally. For a zero-member collection tmp is empty, so the guard belongs on both branches.

Fix

if (tmp.length !== 0) {
  if (isKeyOperator(operator)) {
    result.push(encodeUnreserved(key) + '=' + tmp.join(','));
  } else {
    result.push(tmp.join(','));
  }
}

Verification

• The vendored official RFC 6570 suite's "Empty Variables" cases (extended-tests.json) now pass; spec-examples-by-section.json stays at 117/117 and spec-examples.json at 64/64 — zero regression.
• Two existing assertions in test/url-template-test.js that encoded the old behavior ({?emptylist} → ?emptylist=, {?emptyobject} → ?emptyobject=) are corrected to '', alongside new &/; cases. Note these contradicted the same file's own {?emptylist*} → '' assertion.