Remove nullable getter for ResponsiveValue

[StrDragos]

Remove the null possibility of a ReponsiveValue on the value getter. It is not needed since ResponsiveValue also receives a default value.

Will also address: ResponsiveValue.value should non-nullable #119

[rayliverified]

Thank you for the PR

[rayliverified]

#119

[rayliverified]

Really interesting API decisions with ResponsiveValue.

The choice was between making defaultValue required which would then allow value to inherit the nullability of the generic type. Or, keep defaultValue optional which necessitates value being optional as well, even if a nonnullable type is specified.

The latter was chosen for 2 reasons.

1. defaultValue is a convenience field and should not be required. If there are a lot of conditions, it's better for readability to have the defaultValue at the front rather than behind ??.
2. The Flutter convention for "default" or optional values is ??. ResponsiveValue().value ?? defaultValue is more in line with Flutter conventions.

The drawback with this approach is an explicit nonnull cast ResponsiveValue().value! is required. This isn't completely a drawback as it forces the developer to think about nullability and default values.

[tjarvstrand]

The Flutter convention for "default" or optional values is ??. ResponsiveValue().value ?? defaultValue is more in line with Flutter conventions.

The problem with this approach is that if you pass the ResponsiveValue down the tree, the default value has to be known in every place that it is used.