fix(ldap/auth): handle nil Auth struct when authentication fails

[ecrupper]

// call to get a user token
user, err := vault.Logical().Write(path, options)
if err != nil {
	return nil, fmt.Errorf("unable to get user token: %w", err)
}

The above call does not return an error when the path exists but the password is wrong.

This results in a panic when trying to set the Vault token in the client.

panic: runtime error: invalid memory address or nil pointer dereference

A simple nil check on the Auth struct should suffice.

[codecov]

Codecov Report

Merging #428 (f4b9aad) into main (cc2ce1d) will decrease coverage by 0.42%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #428      +/-   ##
==========================================
- Coverage   42.10%   41.69%   -0.42%     
==========================================
  Files           7        7              
  Lines         304      307       +3     
==========================================
  Hits          128      128              
- Misses        158      161       +3     
  Partials       18       18              

Files	Coverage Δ
vault/vault.go	55.71% <0.00%> (-2.50%)	⬇️