clarify empty actor state == EMPTY_ARR_CID in state tree

Author: raulk

FIPS/fip-0055.md

@@ -165,7 +165,7 @@ For more details, refer to [FIP-0048].
 
 #### State
 
-This actor has no state.
+This actor has no state. The actor's `Head` field in the state tree is the `EMPTY_ARR_CID` static value, defined as the CID of an empty CBOR byte string.
 
 #### `Constructor` (method number `1`)
 
@@ -328,7 +328,7 @@ None.
 
 #### State
 
-None.
+This actor has no state. The actor's `Head` field in the state tree is the `EMPTY_ARR_CID` static value, defined as the CID of an empty CBOR byte string.
 
 ## Design Rationale