Skip evaluation of a Modifying block iff it doesn't do anything

The `Modifying() { }` construct always evaluates even if the block is empty.

With TextKit integration, this means

1. an undo group is being started (and ended)
2. `NSTextView.shouldChangeText(in:replacementString:)` and `didChangeText()` are being run to guard against unwanted changes

With syntax highlighting in the text storage, you may end up processing the text for what's essentially a no-op.

## How to test

To get an empty block, use a for-loop to trigger the `buildArray` path of the result builder, but without any actual iterations:

```swift
Modifying(selectedRange) { _ in
for _ in 0 ..< 0 {
Insert(0) { "loop never runs" }
}
}
```

I'm not sure whether we can figure out at all whether a result builder produces nothing (i.e. empty array).

## Complete test case

This test fails with a thrown error at `try buffer.evaluate` because the text view doesn't permit changes in the range.

This should not be a problem, because the range is not actually changed.

```swift
func testModifying_EmptyLoopBlock_SkipsEvaluation() throws {
class TextViewSpy: NSTextView {
var didCallShouldChangeText = false
var didCallDidChangeText = false

override func shouldChangeText(in affectedCharRange: NSRange, replacementString: String?) -> Bool {
didCallShouldChangeText = true
return false // Would abort modification an error
}

override func didChangeText() {
didCallDidChangeText = true
}
}

let textViewSpy = TextViewSpy()
textViewSpy.string = "Lorem ipsum."
let buffer = NSTextViewBuffer(textView: textViewSpy)
let selectedRange: SelectedRange = .init(location: 6, length: 5)

assertBufferState(buffer, "Lorem ipsum.ˇ")

try buffer.evaluate {
Modifying(selectedRange) { _ in
for _ in 0 ..< 0 {
Insert(0) { "loop never runs" }
}
}
}

XCTAssertFalse(textViewSpy.didCallShouldChangeText)
XCTAssertFalse(textViewSpy.didCallDidChangeText)
}
```