ShellPkg/For: Remove redundant null/empty checks on ArgSetWalker

Fix CodeQL cpp/redundant-null-check-simple warning in
ShellPkg/Library/UefiShellLevel1CommandsLib/For.c at line 571 by
removing redundant null and empty-string checks on ArgSetWalker. The
outer guard at line 570 ensures that ArgSetWalker is non-NULL and
non-empty, so the inner condition only needs
ShellIsValidForNumber(ArgSetWalker).

Signed-off-by: Mingjie Shen <shen497@purdue.edu>