Make createUser id parameter optional

[jenow]

What does this PR do?

Make createUser id parametre optional as it is in the API.

How should this be manually tested?

kuzzle.security.createUser(null, {
    content: {
      profileIds: ['default']
    },
    credentials: {
      local: {
        username: 'auto',
        password: 'auto'
      }
    }
  });

It should create a user with a generated id.

[codecov]

Codecov Report

Merging #408 into 6-dev will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            6-dev    #408      +/-   ##
=========================================
- Coverage   96.31%   96.3%   -0.01%     
=========================================
  Files          32      32              
  Lines        1519    1517       -2     
=========================================
- Hits         1463    1461       -2     
  Misses         56      56

Impacted Files	Coverage Δ
src/controllers/security/index.js	99.58% <ø> (-0.01%)	⬇️

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