cmd/abigen: require either `--abi` or `--combined-json` (#31045)

This PR addresses issue #30768 , which highlights that running
cmd/abigen/abigen --pkg my_package example.json (erroneously omitting
the --abi flag) generates an empty binding, when it should fail
explicitly.

---------

Co-authored-by: jwasinger <j-wasinger@hotmail.com>

cmd/abigen/main.go

@@ -98,6 +98,9 @@ func abigen(c *cli.Context) error {
 	if c.String(pkgFlag.Name) == "" {
 		utils.Fatalf("No destination package specified (--pkg)")
 	}
+	if c.String(abiFlag.Name) == "" && c.String(jsonFlag.Name) == "" {
+		utils.Fatalf("Either contract ABI source (--abi) or combined-json (--combined-json) are required")
+	}
 	var lang bind.Lang
 	switch c.String(langFlag.Name) {
 	case "go":