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/P4Types.swift

@@ -141,8 +141,11 @@ public struct P4QualifiedType: CustomStringConvertible {
     return self.base_type
   }
 
-  public func def() -> P4Value {
-    return P4Value(self.base_type.def(), self)
+  public func def() -> P4Value? {
+    if let default_value = self.base_type.def() {
+      return P4Value(default_value, self)
+    }
+    return .none
   }
 
   public func eq(_ rhs: P4QualifiedType) -> Bool {