Allow resource extension's endpoint_url to be imported as null

[cjgajard]

Closes #869

Acceptance tests introduced...

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -count=1 -run TestAccPagerDutyExtension_import -timeout 120m
?       github.com/PagerDuty/terraform-provider-pagerduty       [no test files]
testing: warning: no tests to run
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerduty     0.541s [no tests to run]
=== RUN   TestAccPagerDutyExtension_import
--- PASS: TestAccPagerDutyExtension_import (19.72s)
=== RUN   TestAccPagerDutyExtension_import_WithoutEndpointURL # 👈 New acceptance introduce to protect from a regression
--- PASS: TestAccPagerDutyExtension_import_WithoutEndpointURL (24.18s)
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerdutyplugin       44.780s
testing: warning: no tests to run
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/util  0.625s [no tests to run]

[imjaroiswebdev]

LGTM 💪🏽 🎉 Thank you @cjgajard