r/aws_amplify_app: fix removal of environment_variables (#39397)

Previously existing `environment_variables` were removed by sending a map with an empty key/value pair. The `UpdateApp` API appears to have added restrictions around keys with empty values, so this approach began to fail the corresponding acceptance test. After reporting the issue to the AWS Go SDK V2, the new method for removing environment variables is to pass a key containing a single space character and an empty string value.

```console
% make testacc PKG=amplify TESTS=TestAccAmplify_serial/App/EnvironmentVariables
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.22.7 test ./internal/service/amplify/... -v -count 1 -parallel 20 -run='TestAccAmplify_serial/App/EnvironmentVariables'  -timeout 360m

--- PASS: TestAccAmplify_serial (301.10s)
    --- PASS: TestAccAmplify_serial/App (301.10s)
        --- PASS: TestAccAmplify_serial/App/EnvironmentVariables (301.10s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/amplify    307.329s
```

.changelog/39397.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+resource/aws_amplify_app: Fix failure when unsetting the `environment_variables` argument
+```

internal/service/amplify/app.go

@@ -545,7 +545,10 @@ func resourceAppUpdate(ctx context.Context, d *schema.ResourceData, meta interfa
 			if v := d.Get("environment_variables").(map[string]interface{}); len(v) > 0 {
 				input.EnvironmentVariables = flex.ExpandStringValueMap(v)
 			} else {
-				input.EnvironmentVariables = map[string]string{"": ""}
+				// To remove environment variables, set the key to a single space
+				// character and the value to an empty string.
+				// Ref: https://github.com/aws/aws-sdk-go-v2/issues/2788
+				input.EnvironmentVariables = map[string]string{" ": ""}
 			}
 		}