Kaniko fails on empty argument with quotes in Dockerfile after 5bbb40e

[denis111]

Actual behavior
Kaniko fails on empty argument with quotes in Dockerfile like:
ARG VARIANT=""
and also empty --build-arg VARIANT=

Expected behavior
Build Dockerfile

To Reproduce
Steps to reproduce the behavior:

1. Use the latest kaniko image.
2. Create Dockerfile with any empty argument but with quotes.

Additional Information

• Dockerfile
  ARG VARIANT=""
  ARG VERSION=2
  FROM registry.example.com/image${VARIANT}:${VERSION}

• Build Context

• Kaniko Image (fully qualified with digest)
  any image after 5bbb40e commit

Triage Notes for the Maintainers

It didn't happen before 5bbb40e commit

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko
Please check if this error is seen when you use --cache flag
Please check if your dockerfile is a multistage dockerfile

Log:

val ""
val 2
panic: runtime error: slice bounds out of range

goroutine 1 [running]:
github.com/GoogleContainerTools/kaniko/pkg/dockerfile.stripEnclosingQuotes(0xc0004cd810, 0x2, 0x2, 0x1, 0xc0004cd810, 0x2, 0x2, 0x0)
	/go/src/github.com/GoogleContainerTools/kaniko/pkg/dockerfile/dockerfile.go:137 +0x4ad
github.com/GoogleContainerTools/kaniko/pkg/dockerfile.Parse(0xc0008ee000, 0xfa, 0x2fa, 0xfa, 0x2fa, 0x0, 0x0, 0x0, 0xc000104990, 0x0, ...)
	/go/src/github.com/GoogleContainerTools/kaniko/pkg/dockerfile/dockerfile.go:115 +0xfd
github.com/GoogleContainerTools/kaniko/pkg/dockerfile.Stages(0x30f5f20, 0x6502681, 0x30f5980, 0x4fecae, 0x28, 0x203000)
	/go/src/github.com/GoogleContainerTools/kaniko/pkg/dockerfile/dockerfile.go:56 +0x12c
github.com/GoogleContainerTools/kaniko/pkg/executor.DoBuild(0x30f5f20, 0x1, 0x0, 0x0, 0xc0005b6500)
	/go/src/github.com/GoogleContainerTools/kaniko/pkg/executor/build.go:426 +0xcf
github.com/GoogleContainerTools/kaniko/cmd/executor/cmd.glob..func2(0x30e5580, 0xc000105dd0, 0x0, 0x9)
	/go/src/github.com/GoogleContainerTools/kaniko/cmd/executor/cmd/root.go:92 +0x11b
github.com/GoogleContainerTools/kaniko/vendor/github.com/spf13/cobra.(*Command).execute(0x30e5580, 0xc00003a0b0, 0x9, 0x9, 0x30e5580, 0xc00003a0b0)
	/go/src/github.com/GoogleContainerTools/kaniko/vendor/github.com/spf13/cobra/command.go:766 +0x2ae
github.com/GoogleContainerTools/kaniko/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0x30e5580, 0x17a26bd, 0xc0005e7f88, 0xc0000461d8)
	/go/src/github.com/GoogleContainerTools/kaniko/vendor/github.com/spf13/cobra/command.go:852 +0x2ec
github.com/GoogleContainerTools/kaniko/vendor/github.com/spf13/cobra.(*Command).Execute(...)
	/go/src/github.com/GoogleContainerTools/kaniko/vendor/github.com/spf13/cobra/command.go:800
main.main()
	/go/src/github.com/GoogleContainerTools/kaniko/cmd/executor/main.go:26 +0x2e