shell: correct z_shell_spaces_trim when len equal to zero

Addresses a potential issue when the string contains only '\0',
which results in z_shell_strlen(str) returning len as 0.
Since the for-loop expects len >= 1, the function now immediately
returns when len == 0.
As the null check for 'str' is already handled inside
z_shell_strlen(), an additional check for 'str' is not needed.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>