shell: fix Tab crashing issue

Commit 770f232a removes an array
termination from the shell_make_argv function. This function was
called from two functions: execute and tab_handle. In the mentioned
commit termination was added only in the execute function.
As a result calling tab_handle function was causing a segmentation
fault. This fix adds array termination to the tab_handle function.

Fixes #28434

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordisemi.no>