fix: fix ge val nil panic

[HeyJavaBean]

What type of PR is this?

fix

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• The description of this PR title is user-oriented and clear enough for others to understand.
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try:

struct FieldFilterI64 {
    
     1: optional i64 Gte

    
     2: optional i64 f1 ( vt.ge="$Gte")
     3: optional i64 f2 ( vt.ge="2")

     4: required i64 GteR
     5: optional i64 f3 ( vt.ge="$GteR")

}

and

var lte int64 = 10
	ff := &c.FieldFilterI64{
		Gte: nil,
		F1: &lte,
		F2: &lte,
		F3: &lte,
	}

	err := ff.IsValid()
	if err != nil {
		panic(err)
	}

to show the panic.

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