beacon/config: ignore nil values in config file (ethereum#33065)

YAML supports leaving out the value, so we should handle this condition
in our limited parser.

Author: klim0v

beacon/params/config.go

@@ -103,6 +103,9 @@ func (c *ChainConfig) LoadForks(file []byte) error {
 	epochs["GENESIS"] = 0
 
 	for key, value := range config {
+		if value == nil {
+			continue
+		}
 		if strings.HasSuffix(key, "_FORK_VERSION") {
 			name := key[:len(key)-len("_FORK_VERSION")]
 			switch version := value.(type) {

beacon/params/config_test.go

@@ -15,6 +15,9 @@ ALTAIR_FORK_EPOCH: 1
 EIP7928_FORK_VERSION: 0xb0000038
 EIP7928_FORK_EPOCH: 18446744073709551615
 
+EIP7XXX_FORK_VERSION: 
+EIP7XXX_FORK_EPOCH: 
+
 BLOB_SCHEDULE: []
 `
 	c := &ChainConfig{}