compiler, runtime: Make Default Values Optional

For types, make it optional to return a default value. While default
values for some types are meaningful, they are not meaningful for all
types (e.g., Action, Control, Parser, Parser state, etc.).

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Sources/Common/Protocols.swift

@@ -36,7 +36,7 @@ public protocol EvaluatableStatement {
 
 public protocol P4Type: CustomStringConvertible {
   func eq(rhs: any P4Type) -> Bool
-  func def() -> P4DataValue
+  func def() -> P4DataValue?
 }
 
 public protocol P4DataValue: CustomStringConvertible {