diff(..., 'year'|'month', true) returns 0 instead of NaN for an Invalid Date operand

Describe the bug

monthDiff in src/utils.js silently swallows NaN into 0 via its final || 0:

const monthDiff = (a, b) => {
  if (a.date() < b.date()) return -monthDiff(b, a)
  const wholeMonthDiff = ((b.year() - a.year()) * 12) + (b.month() - a.month())
  const anchor = a.clone().add(wholeMonthDiff, C.M)
  const c = b - anchor < 0
  const anchor2 = a.clone().add(wholeMonthDiff + (c ? -1 : 1), C.M)
  return +(-(wholeMonthDiff + ((b - anchor) / (c ? (anchor - anchor2) :
    (anchor2 - anchor)))) || 0)
}

This is used by .diff(other, 'year'|'month', true). When either side of the diff is an Invalid Date, the intermediate arithmetic naturally produces NaN, but the || 0 at the end turns that into a plausible-looking 0 instead of propagating the invalidity -- so .diff() silently reports "0 months apart" for a comparison that is actually meaningless.

Reproduction

const dayjs = require('dayjs');

const invalid = dayjs('not a date');
const now = dayjs();

invalid.diff(now, 'month', true); // 0 -- should be NaN
invalid.diff(now, 'year', true);  // 0 -- should be NaN

// Compare with day/hour/etc units, which use raw ms diff and correctly propagate NaN:
invalid.diff(now, 'day', true);   // NaN (correct)
invalid.diff(now, 'hour', true);  // NaN (correct)

Expected behavior

.diff(..., 'year'|'month', true) should return NaN when either operand is an Invalid Date, consistent with every other unit (day, hour, minute, second, week, quarter all correctly return NaN already, since they compute from the raw millisecond diff = this - that without an || 0 fallback).

How I found this

While looking at #3006 / PR #3024 (duration.humanize() returning "a month" for dayjs.duration(NaN)) -- that bug's actual root cause is exactly this monthDiff fallback (humanize() ends up diffing an Invalid-Date-wrapping instant against "now" in Y/M units, and monthDiff's || 0 turns the resulting NaN into 0, which the relativeTime threshold loop then reads as "0 months" and prints "a month"). PR #3024 already fixes the humanize() symptom directly at that boundary, so this issue is just the underlying monthDiff behavior itself, which can still surface through any other code path that calls .diff(..., 'year'|'month') with an invalid operand (not just via duration).

Information

• Day.js Version: 1.11.22 (current dev)