do not return undefined colors when splitting brand

[gordonwoodhull]

fixes #13369

E.g. the light brand for

brand:
  color:
    background:
      light: '#f3f3f8'
      dark: '#0f0412'
    foreground:
      light: '#113322'
      dark: '#f8f8f8'
    primary:
      dark: '#20bbff'

should not have primary: undefined

Will backport to v1.8 and changelog there.

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