Fix not defined env APP_URL on new setups

[mynameisbogdan]

1. Why is this change necessary?

To prevent the error Environment variable not found: "APP_URL". on new setups.

2. What does this change do, exactly?

Sets a default value of http://localhost for APP_URL

3. Describe each step to reproduce the issue or behaviour.

Run bin/console system:setup

4. Please link to the relevant issues (if any).

shopware/production#101

5. Checklist

• I have written tests and verified that they fail without my change
• I have squashed any insignificant commits
• I have created a changelog file with all necessary information about my changes
• I have written or adjusted the documentation according to my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfil them.

[CLAassistant]

All committers have signed the CLA.

[J-Rahe]

Hey @mynameisbogdan, could you please sign the cla?

[mynameisbogdan]

hello @J-Rahe, ok, just did.

Thank you.

[pweyck]

I've recently added a default of ''. Could you change it here instead?

[mynameisbogdan]

@pweyck the bug it's fixed with that empty string as well. I created a new PR #1859 regarding just the change of the default value to http://localhost.

[mynameisbogdan]

I created a new PR #1859 regarding just the change of the default value to http://localhost.