ReadOnlySpan created from a null array does not return a null ReadOnlySpan

### Type of issue

Typo

### Description

For the `ReadOnlySpan(T[]?)` constructor that can take a null array, the doc says `If the array is null, this constructor returns a null ReadOnlySpan.`

However, the method instead creates an empty span. A span, being a `ref struct` cannot be nullable. It should say "This method returns default when array is null." as described in the other constructor `ReadOnlySpan(T[], Int32, Int32)`.

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.readonlyspan-1.-ctor?view=net-8.0

### Content source URL

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System/ReadOnlySpan`1.xml

### Document Version Independent Id

d7b1c5b6-cdec-e856-0d48-533861086507

### Article author

@steveharter