gohcl: skip encoding additional nil fields

[jedevc]

This patch skips encoding nil slices, maps and interfaces, in addition to the already skipped nil pointers.

The scenario I've run into this in is something like the following:

type X struct {
	A *string  `hcl:"a"`
	B []string `hcl:"b"`
}

...

f := hclwrite.NewEmptyFile()
block := gohcl.EncodeAsBlock(X{}, "test")
f.Body().AppendBlock(block)
fmt.Println(string(f.Bytes()))

Without the patch I get:

test {
  b = null
}

While with the patch, I get:

test {
}

Hopefully this small change is something worth incorporating!

[hashicorp-cla]

All committers have signed the CLA.

[jedevc]

Is there anything currently blocking this?

[yuri-tceretian]

I think the change does a good thing. Any chance it can be merged?