[CDTOOL-1225] Correct keepalive_time Drift

[rcaril]

Change summary

This PR fixes a bug where the keepalive_time backend attribute was incorrectly being set to 0 in state when a value wasn't being set.

A more in depth explanation can be found here: https://fastly.atlassian.net/browse/CDTOOL-1225

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New Feature Submissions:

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--- PASS: TestAccFastlyServiceVCLBackend_Minimal (9.28s)
PASS
ok   

Changes to Core Features:

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User Impact

• What is the user impact of this change?

Users should not longer see drift when updating backend objects in Terraform due to inconsistency between the keepalive_time attribute being changed from 0 -> null.

[philippschulte]

LGTM. Thanks for doing this!