Fix panic I see when upgrading to 0.6.6

[bobtfish]

Check if the policy is nil or not before type casting it

Otherwise I get this error:

panic: interface conversion: interface is nil, not string
2015/11/04 04:02:04 [DEBUG] terraform-provider-aws:
2015/11/04 04:02:04 [DEBUG] terraform-provider-aws: goroutine 263 [running]:
2015/11/04 04:02:04 [DEBUG] terraform-provider-aws: github.com/hashicorp/terraform/builtin/providers/aws.func·123(0x0, 0x0, 0x0, 0x0)
2015/11/04 04:02:04 [DEBUG] terraform-provider-aws:     /root/build/src/github.com/hashicorp/terraform/builtin/providers/aws/resource_aws_sns_topic.go:50 +0x64
2015/11/04 04:02:04 [DEBUG] terraform-provider-aws: github.com/hashicorp/terraform/helper/schema.schemaMap.diffString(0xc2090603f0, 0x112aab0, 0x6, 0xc20906c5b0, 0xc20910b050, 0xc2091443c0, 0xc20906c400, 0x0, 0x0)

[catsby]

Great catch! Do you think we should just immediately return "" if v is nil?

[apparentlymart]

I remember we were discussing this on some other ticket somewhere (I couldn't find it after a quick search) and a bunch of us concluded and agreed that StateFunc ought to be called only for non-null values, after a bug very similar to this. We couldn't find ause-case for transforming nulls, especially since the callback is required to return a string anyway and so couldn't normalize to null.

So +1 from me on that!

[catsby]

Continued in #3777 where I make a small change, thanks!

[ghost]

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