Optimize Element.OnPropertyChanged

[eerhardt]

• Don't foreach over the empty collection ChildrenNotDrawnByThisElement. The bulk of Element subclasses don't provide a value. Instead just return null.

This is a pretty minor, low-level optimization. But it shows to save 10% off setting simple properties.

Benchmark Results

Before

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
SetProperty	1.736 ms	0.0068 ms	0.0057 ms	-	-	-	665 B

After

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
SetProperty	1.546 ms	0.0151 ms	0.0133 ms	-	-	-	665 B

[jonathanpeppers]

This one looks straightforward to me, a small improvement for everything that subclasses Element.