Closes #2350

New rule @angular-eslint/template/prefer-at-empty will find code in templates where the @empty block of a @for block could be used instead of using an @if block.

For example, it will find this:

@for (item of items; track $index) {
    {{ item }}
}
@if (items.length === 0) {
  No items
}

and change it to this:

@for (item of items; track $index) {
    {{ item }}
} @empty {
  No items
}

I originally wrote this to just report the problems and not provide a fix, however because there are a few different scenarios that it detects, it wasn't always obvious how to fix the problem. I considered using the message to explain what needs to change, but then I figured if it's going to explain what needs to be changed, why not just change it!

The fixes change the code enough to make the problem go away, but it doesn't do any formatting, so the expected output in the tests can look a bit messy, but a formatter like Prettier will fix that up nicely.

There are six different scenarios that can be detected. Of course, in each scenario, the collection used in the for block must be the same collection used in the @if blocks. The scenarios are:

1. @for followed by an "is empty" @if:

@for (item of items; track $index) {
}
@if (items.length === 0) {
}

2. An "is empty" @if followed by @for:

@if (items.length === 0) {
}
@for (item of items; track $index) {
}

3. An "is not empty" @if with an @else, where the @if branch contains a @for:

@if (items.length > 0) {
    @for (item of items; track $index) {
    }
} @else {
}

3. An "is empty" @if with an @else, where the @else branch contains a @for:

@if (items.length === 0) {
} @else { 
    @for (item of items; track $index) {
    }
}

5. An "is not empty" @if with a @for, followed by an "is empty" @if:

@if (items.length > 0) {
    @for (item of items; track $index) {
    }
}
@if (items.length === 0) {
}

6. An "is empty" @if followed by an "is not empty" @if with a @for:

@if (items.length === 0) {
}
@if (items.length > 0) {
    @for (item of items; track $index) {
    }
}