Fix empty ssh key importing in ldap

[zeripath]

The current implementation assumes that any LDAP will either return a valid SSH public key or none at all. It appears that some LDAPs will simply return "" which will cause an out of bounds in our code.

This code protects the importation of public keys by checking that these can be split in to a space separated string (presumably key type and key content - checked later). If the string provided cannot do this it is ignored.

Many thanks to @silverwind for their excellent bug report.

Fix #5975

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@44d6a90). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #5984   +/-   ##
=========================================
  Coverage          ?   38.79%           
=========================================
  Files             ?      330           
  Lines             ?    48731           
  Branches          ?        0           
=========================================
  Hits              ?    18904           
  Misses            ?    27088           
  Partials          ?     2739

Impacted Files	Coverage Δ
models/user.go	48.89% <100%> (ø)

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[techknowlogick]

@zeripath please backport 😄

[zeripath]

I've finally put up the backport as #6009