check mode enabled: explicit error when PostgreSQL is missing

check_mode: fail with an explicit message when _pg_version is empty. Without this change the following error occurs:

TASK [postgresql : get PostgreSQL version]
duckcorp-infra/ansible/roles/postgresql/tasks/main.yml:25
ok: [Orthos] => {
    "changed": false,
    "cmd": "pg_lsclusters -h | awk '{ if ( $2 = \"main\" ) { print $1 } }'",
    "rc": 0,
    "stderr": "/bin/sh: 1: pg_lsclusters: not found",
    "stdout": "",
}

TASK [postgresql : define PostgreSQL version]
duckcorp-infra/ansible/roles/postgresql/tasks/main.yml:36
ok: [Orthos] => {
    "ansible_facts": {"_pg_version": ""},
}

TASK [postgresql : debug]
duckcorp-infra/ansible/roles/postgresql/tasks/main.yml:40
ok: [Orthos] => {
    "msg": "The detected PostgreSQL version is: "
}

[...]

TASK [postgresql : Create PostgreSQL Configuration Extra Directory]
duckcorp-infra/ansible/roles/postgresql/tasks/main.yml:65
fatal: [Orthos]: FAILED! => {
    "msg": "The conditional check '_pg_version is version('9.3', '>=')' failed.
            The error was: Version comparison: 'LooseVersion' object has no attribute 'version'"
}