let getDefaultNamespace() returns "default" if nothing is found.

[jmuk]

https://github.com/istio/issues/issues/110 reminds me that istioctl can set empty namespace for mixer configs, which will be delivered directly and therefore it can cause same error of "empty namespace".

When the obtained namespace happens to be empty, I think it should return the default value.

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #1637

Special notes for your reviewer:

Release note:

none

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[jmuk]

/assign @ayj

[codecov[bot]]

Codecov Report

Merging #1638 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1638   +/-   ##
=======================================
  Coverage   83.28%   83.28%           
=======================================
  Files          52       52           
  Lines        6546     6546           
=======================================
  Hits         5452     5452           
  Misses        890      890           
  Partials      204      204

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[jmuk]

/retest

[andraxylia]

@jmuk could you please cherry-pick this into 0.2? Our request routing instructions no longer work.