fix(Layouts): fix propTypes error in StickyLayout

[kalzoo]

since fixed is an enum type, sending false instead of undefined causes a proptypes error:

Warning: Failed prop type: Invalid prop `fixed` of value `false` supplied to `Menu`, expected one of ["left","right","bottom","top"].
    in Menu (created by Context.Consumer)
    in HeaderFooter (created by Layout)
    in Query
    in Unknown (created by Route)
    in Route (created by withRouter())
    in withRouter() (created by Layout)
    in Layout (created by App)
    in Router (created by BrowserRouter)
    in BrowserRouter (created by App)
    in ApolloProvider (created by App)
    in App

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[codecov-io]

Codecov Report

Merging #3261 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3261   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files         169      169           
  Lines        2802     2802           
=======================================
  Hits         2800     2800           
  Misses          2        2

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