style(preset-built-in): Condition 'windowInitialVars' is always true

[xjh22222228]

Condition 'windowInitialVars' is always true, remove {}

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• any feature?
• close https://github.com/umijs/umi/ISSUE_URL

[codecov]

Codecov Report

Merging #6408 (efb8ffa) into master (0bb4290) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6408   +/-   ##
=======================================
  Coverage   84.73%   84.73%           
=======================================
  Files         157      157           
  Lines        3564     3564           
  Branches      975      974    -1     
=======================================
  Hits         3020     3020           
  Misses        534      534           
  Partials       10       10           

Impacted Files	Coverage Δ
...ilt-in/src/plugins/features/ssr/templates/utils.ts	100.00% <100.00%> (ø)

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[ycjcl868]

Object.keys(var || {}) 保守写法，避免 var undefined 造成 error

[xjh22222228]

Object.keys(var || {}) 保守写法，避免 var undefined 造成 error

不对呀， windowInitialVars 始终是对象，怎么会为 undefined, 而且是个常量。