Fix handling of empty input in describe-variable and describe-symbol

Fix handling of empty input in describe-variable and describe-symbol

* lisp/help-fns.el (describe-variable): Use 'user-error' like in
'describe-function'.
(describe-symbol): Use empty string for arg SYMBOL when input is empty
and there is no default value.
This allows to signal the error "You didn't specify a function or variable"
instead of displaying help about the symbol 'nil' on empty input.
OTOH, still allows to see help about 'nil' when the input is "nil".