triedb/pathdb: fix tester generator (ethereum#30972)

This change fixes is a rare bug in test generator: If the run is very unlucky it
can use `modifyAccountOp` / `deleteAccountOp` without creating any
account, leading to have a trie root same as the parent.

This change makes the first operation always be a creation.

Author: ceyonur

triedb/pathdb/database_test.go

@@ -222,7 +222,12 @@ func (t *tester) generate(parent common.Hash) (common.Hash, *trienode.MergedNode
 		dirties = make(map[common.Hash]struct{})
 	)
 	for i := 0; i < 20; i++ {
-		switch rand.Intn(opLen) {
+		// Start with account creation always
+		op := createAccountOp
+		if i > 0 {
+			op = rand.Intn(opLen)
+		}
+		switch op {
 		case createAccountOp:
 			// account creation
 			addr := testrand.Address()