Allow 'completion-preview-require-minimum-symbol-length' to be nil

Allow 'completion-preview-require-minimum-symbol-length' to be nil

With some completion backends, completion preview is useful not only
after a partial symbol, but also after punctuation and other non-symbol
characters.  For example, in C code it's helpful to display the
completion preview for struct members when point is after 'foo->'.
Provide an option to skip the check for minimum symbol length at point
in order to support this use case.

* lisp/completion-preview.el
(completion-preview-minimum-symbol-length): Mention possible nil
value in type and docstring.
(completion-preview-require-minimum-symbol-length): Skip check
if 'completion-preview-minimum-symbol-length' is nil.