fix session password

[butonic]

Found this while working on kerberos. Password should be null instead of '' IIUC that null triggers \OC\Authentication\Exceptions\PasswordlessTokenException to make sessions without a password work properly. No idea if ther is magic in core that converts '' to null ... oracle certainly has that magic ... so no idea how this affects the overall auth with oauth2.

• Also needs PHPDoc update in core

[codecov]

Codecov Report

Merging #129 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #129   +/-   ##
=========================================
  Coverage     82.12%   82.12%           
  Complexity      199      199           
=========================================
  Files            21       21           
  Lines           705      705           
=========================================
  Hits            579      579           
  Misses          126      126

Impacted Files	Coverage Δ	Complexity Δ
lib/AuthModule.php	88.88% <100%> (ø)	8 <0> (ø)	⬇️

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