This reverts emacs-lsp#2112.

The intent of `lsp--suggest-project-root` is not to always return non-nil but
instead return nil when no good suggestion is found and let the caller decide
what to do in such case. This can be seen in 2 different ways:

1. All calls except in `lsp--calculate-root` (see 2 below) use the following
form which demonstrates that the function should sometimes return nil.

    ```
    (or (lsp--suggest-project-root) default-directory)
    ```

2. We find the following snippet in `lsp--calculate-root` which demonstrates
that the function should sometimes return nil (otherwise `unless
lsp-auto-guess-root` could be removed since always true).

    ```
    (or
     (when lsp-auto-guess-root
       (lsp--suggest-project-root))
     (lsp-find-session-folder session file-name)
     (unless lsp-auto-guess-root
    ```