subsys: shell: removing troubleshooting assert

Removing assert that was crashing shell in allowed scenario.
For example the Tab button is pressed when command buffer is empty.

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

subsys/shell/shell.c

@@ -148,7 +148,6 @@ static void cmd_get(const struct shell_cmd_entry *command, size_t lvl,
 		    struct shell_static_entry *d_entry)
 {
 	assert(entry != NULL);
-	assert(command != NULL);
 	assert(d_entry != NULL);
 
 	if (lvl == SHELL_CMD_ROOT_LVL) {