[BUG] Make PhotoViewController.scale should not be null

[alestiago]

Describe the bug
Currently, PhotoViewController.scale is nullable. However, this value should not be null according to @renancaraujo.

Although, I do not envision how this value can be nullable. If so, docs should be added to document under what scenarios this values is nullable.

To Reproduce
(

photo_view/lib/src/controller/photo_view_controller.dart

Line 56 in 7e8c335

late double? scale;

)

What is the current behavior?
PhotoViewController.scale is double? == true
PhotoViewController.scale is double == false

Expected behavior
PhotoViewController.scale is double == true

Screenshots
N/a

Which versions of Flutter/Photo View, and which browser / OS are affected by this issue? Did this work in previous versions of Photo View?

Photo View ^0.14.0

Flutter doctor

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.0.5, on macOS 12.2.1 21D62 darwin-arm, locale
    en-GB)
[✓] Android toolchain - develop for Android devices (Android SDK version 33.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 13.4.1)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2021.2)
[✓] VS Code (version 1.70.2)
[✓] Connected device (3 available)
[✓] HTTP Host Availability

• No issues found!

[alestiago]

(cc: @renancaraujo)