fix Affix/index.tsx while offsetTop === 0 and offsetBottom is numberic

[qhxin]

Fix: if offsetTop === 0, it will get undefined, if offsetBottom is type of number, offsetMode will be { top: false, ... }

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[codecov]

Codecov Report

Merging #10566 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master   #10566   +/-   ##
=======================================
  Coverage    86.4%    86.4%           
=======================================
  Files         196      196           
  Lines        4795     4795           
  Branches     1339     1339           
=======================================
  Hits         4143     4143           
  Misses        649      649           
  Partials        3        3

Impacted Files	Coverage Δ
components/affix/index.tsx	66.66% <0%> (ø)	⬆️

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[afc163]

Could you add a test case for this?