fix(a380/mfd): FLT NBR is now displayed as a mandatory input field

[jet2code]

Fixes #9752

Summary of Changes

• Set atcCallsign's initial value to null to ensure it met the criteria for displaying mandatory boxes in InputField
• This displays the FLT NBR input field as mandatory. CPNY RTE was left as is as it is currently a disabled field.

Screenshots (if necessary)

References

Additional context

Discord username: john.xyz

Testing instructions

1. Open the INIT page
2. Ensure FLT NBR has mandatory input symbology

How to download the PR for QA

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