fix: opsMap must not be nil

Author: tyru

dsl/ops/lookup.go

@@ -4,7 +4,7 @@ import "github.com/vim-volt/volt/dsl/types"
 
 // opsMap holds all operation structs.
 // All operations in dsl/op/*.go sets its struct to this in init()
-var opsMap map[string]types.Op
+var opsMap = make(map[string]types.Op)
 
 // Lookup looks up operator name
 func Lookup(name string) (types.Op, bool) {