Add Hash::dummy to signal default value

We did this in the same spirit of the dummy value that's present in
libstore/path.hh

src/libstore/legacy-ssh-store.cc

@@ -113,7 +113,7 @@ struct LegacySSHStore : public Store
 
             if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 4) {
                 auto s = readString(conn->from);
-                info->narHash = s.empty() ? Hash(htSHA256) : Hash::parseAnyPrefixed(s);
+                info->narHash = s.empty() ? Hash::dummy : Hash::parseAnyPrefixed(s);
                 info->ca = parseContentAddressOpt(readString(conn->from));
                 info->sigs = readStrings<StringSet>(conn->from);
             }

src/libstore/path-info.hh

@@ -100,8 +100,8 @@ struct ValidPathInfo
 
     ValidPathInfo(const ValidPathInfo & other) = default;
 
-    ValidPathInfo(StorePath && path) : path(std::move(path)), narHash(Hash(htSHA256)) { };
-    ValidPathInfo(const StorePath & path) : path(path), narHash(Hash(htSHA256)) { };
+    ValidPathInfo(StorePath && path) : path(std::move(path)), narHash(Hash::dummy) { };
+    ValidPathInfo(const StorePath & path) : path(path), narHash(Hash::dummy) { };
 
     virtual ~ValidPathInfo() { }
 };

src/libutil/hash.cc

@@ -136,6 +136,8 @@ std::string Hash::to_string(Base base, bool includeType) const
     return s;
 }
 
+Hash Hash::dummy(htSHA256);
+
 Hash Hash::parseSRI(std::string_view original) {
     auto rest = original;
 

src/libutil/hash.hh

@@ -105,6 +105,8 @@ public:
         assert(type == htSHA1);
         return std::string(to_string(Base16, false), 0, 7);
     }
+
+    static Hash dummy;
 };
 
 /* Helper that defaults empty hashes to the 0 hash. */