'Nothing' sometimes evalutes to 0, 0.0, False, ChrW(0), or other value type defaults when a null value is expected

Nothing is a target-typed literal whose actual value depends on the type it's being converted to. Most users think of `Nothing` as synonymous with a null value so when this isn't the case it's often a surprise or surprisingly causes undesirable behavior:

```VB.NET
' User thinks x has null value but x has value 0.
Dim x As Boolean? = If(False, 1, Nothing)
```

```VB.NET
' User thinks x has type Integer? (nullable) but x has type Integer (non-nullable).
Dim x = If(False, 1, Nothing)
```

This requires special handling to explicitly casting to produce a null value.