helper/schema: Computed Map fields generate perpetual diff when empty

[phinze]

This is currently breaking a bunch of acceptance tests since we've landed #1443.

For example, TestAccHerokuApp_NukeVars:

--- FAIL: TestAccHerokuApp_NukeVars (6.09s)
        testing.go:131: Step 1 error: After applying this step, the plan was not empty:

                DIFF:

                UPDATE: heroku_app.foobar
                  all_config_vars.#: "" => "<computed>"

                STATE:

                heroku_app.foobar:
                  ID = terraform-test-app
                  all_config_vars.# = 0
                  config_vars.# = 0
                  git_url = [email protected]:terraform-test-app.git
                  heroku_hostname = terraform-test-app.herokuapp.com
                  name = terraform-test-app
                  region = us
                  stack = cedar-14
                  web_url = https://terraform-test-app.herokuapp.com/
FAIL

The issue centers around how helper/schema distinguishes between the following two scenarios:

• Map is empty in config, state is nil, future value of map is unknown (should generate a diff)
• Map is empty in config, state has been written as empty, future value of map is known to be empty (should not generate a diff)

Important locations for relevant behavior:

• (m schemaMap) diffMap
• (r *MapFieldReader) readMap
• (r *MapFieldWriter) writeMap
• (m schemaMap) finalizeDiff

Pushing a branch with a failing test and some debug logging shortly.

[ghost]

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