Fixed docblock in UserInterface::getSalt()

This method can return null, too.
See the line above:

> "This can return null if the password was not encoded using a salt."
merged branch apfelbox/patch-1 (PR #8982)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #8982).

Discussion
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Fixed docblock in UserInterface::getSalt()

This method can return null, too.
See the line above:

> "This can return null if the password was not encoded using a salt."

Commits
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3c7b557 Fixed docblock in UserInterface::getSalt()