[datadog_synthetics_test] Ensure tags are non-empty and castable

[nkzou]

SSIA, bugfix in response to customer ticket

[Drarig29]

Does it mean the tag will be silently ignored if not castable to string?

Can you give an example of something non-castable that the user was passing?

[Drarig29]

LGTM

For context, a customer had tags = ["foo:bar", "", "hello:world"] and it was returning the same error as #1795:

interface {} is nil, not string