Fix type conversion bug

[koikonom]

Description

While iterating over a trigger prefix list sometimes items can be nil.
We are now checking if conversion is possible instead of "blindly"
converting them and potentially causing a crash.

[koikonom]

Closes #183 .

[lafentres]

this looks good to me! tested it locally with this config and compared before/after:

resource "tfe_workspace" "test-workspace" {
  name = "test-workspace"
  organization = tfe_organization.test-org.name
  trigger_prefixes = ["", "test"]
}

it seems like the previous code was assuming that the elements of trigger_prefixes were never just empty strings. it might make sense to add some sort of validation for each element eventually, that way people get a warning that they've provided an improper trigger prefix.