Perform null check before accessing it

[pregress]

Description

Perform the null check on RouteTable.Routes before accessing it with SingleOrDefault.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
    • {Please put the link here}
  • the markdown help files have been regenerated using the commands listed here

[ghost]

Thank you for your contribution pregress! We will review the pull request and get back to you soon.

[msJinLei]

@pregress Thanks for the contribution!
Please add a line to describe your change of this PR in https://github.com/Azure/azure-powershell/blob/05a86d3c19079361ffd103e1167b89e0c4583484/src/Network/Network/ChangeLog.md
BTW, are you a Microsoft team member?

[pregress]

Described the change in the changelog.
No, I'm not a MSFT team member.

[isra-fel]

Hey, src/Network/Network/Generated/RouteTable/Route/AddAzureRmRouteConfigCommand.cs this file is generated so we probably should not update it manually.

@MikhailTryakhov Hey Mikhail the customer is trying to fix a null-reference exception in the generated code of Network. What's you advice here?

[pregress]

Can somebody point me in the right direction, from where the source code is generated?
I'll change it there.

[msJinLei]

Can somebody point me in the right direction, from where the source code is generated? I'll change it there.

src/Network/Network/Generated/RouteTable/Route/AddAzureRmRouteConfigCommand.cs looks generated. Need @MikhailTryakhov to confirm.

[wyunchi-ms]

Please resolve the conflicts

[wyunchi-ms]

/azp run azure-powershell - security-tools

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