fix:kustomize cfg grep with no arguments causes panic

[deszhou]

• Before the Fix

➜  ✗ kustomize cfg grep                                
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
sigs.k8s.io/kustomize/cmd/config/internal/commands.(*GrepRunner).preRunE(0x104cf5ad0?, 0x0?, {0x105ac5f68?, 0x104cf0f8c?, 0x1400018bd00?})
          /Users/denniszh/kustomize/cmd/config/internal/commands/grep.go:69 +0x3bc
github.com/spf13/cobra.(*Command).execute(0x14000287b08, {0x105ac5f68, 0x0, 0x0})
        /Users/denniszh/go/pkg/mod/github.com/spf13/[email protected]/command.go:968 +0x7e8
github.com/spf13/cobra.(*Command).ExecuteC(0x140001e2608)
         /Users/denniszh/go/pkg/mod/github.com/spf13/[email protected]/command.go:1115 +0x344
github.com/spf13/cobra.(*Command).Execute(0x1059bbfa8?)
        /Users/denniszh/go/pkg/mod/github.com/spf13/[email protected]/command.go:1039 +0x1c
main.main()
       /Users/denniszh/kustomize/kustomize/main.go:14 +0x20

• After the Fix

➜ ✗ kustomize cfg grep
Error: missing required argument: QUERY
Usage:
 kustomize cfg grep QUERY [DIR] [flags]

Examples:

   # find Deployment Resources
   kustomize cfg grep "kind=Deployment" my-dir/

   # find Resources named nginx
   kustomize cfg grep "metadata.name=nginx" my-dir/

   # use tree to display matching Resources
   kustomize cfg grep "metadata.name=nginx" my-dir/ | kustomize cfg tree

   # look for Resources matching a specific container image
   kustomize cfg grep "spec.template.spec.containers[name=nginx].image=nginx:1\.7\.9" my-dir/ | kustomize cfg tree

Flags:
     --annotate              annotate resources with their file origins. (default true)
 -h, --help                  help for grep
     --invert-match          Selected Resources are those not matching any of the specified patterns..
 -R, --recurse-subpackages   also print resources recursively in all the nested subpackages (default true)

Global Flags:
     --stack-trace   print a stack-trace on error

Fixes: #5701

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[stormqueen1990]

/ok-to-test /label tide/merge-method-squash

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: deszhou, varshaprasad96

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