Treat an unset CB_* secret as absent, not as an empty value

The cloud workflow sets every CB_* variable in every matrix cell, as
`${{ matrix.cloud-provider == '<x>' && secrets.<VAR> || '' }}`, so a
variable belonging to another cell - or one whose secret is not configured
at all - arrives as an empty string rather than absent. get_env is
os.environ.get, which returns that empty string in preference to the
default, so the test data ends up blank instead of falling back.

Wiring CB_IMAGE_AWS into the workflow in the previous commit made this
reachable: CB_IMAGE_AWS has no secret behind it, so the AWS cell now
exports it as "" and the suite would launch instances with an empty image
id rather than the default this branch just updated. The same hazard
applies to every other CB_* entry; the other providers only escape it
because their secrets happen to be set.

Route the test data through a lookup that treats unset and empty alike.

tests/helpers/__init__.py

@@ -79,6 +79,20 @@ def skipIfPython(op, major, minor):
     return wrap
 
 
+def env_or(varname, default_value):
+    """
+    Environment variable ``varname``, treating unset and empty alike.
+
+    The cloud workflow sets every CB_* variable in every matrix cell, as
+    ``${{ matrix.cloud-provider == '<x>' && secrets.<VAR> || '' }}``, so a
+    variable belonging to another cell - or one whose secret is simply not
+    configured - arrives as an empty string rather than absent. A plain
+    ``os.environ.get`` hands that empty string straight back, silently
+    blanking the test data instead of falling back to the default.
+    """
+    return cb_helpers.get_env(varname) or default_value
+
+
 TEST_DATA_CONFIG = {
     "AWSCloudProvider": {
         # Ubuntu 24.04 LTS, us-east-1, amd64, gp3 (Canonical, 20260714). AMI
@@ -95,33 +109,33 @@ TEST_DATA_CONFIG = {
         #
         # moto does not validate instance-launch AMI ids, so the mock
         # provider is indifferent to this value.
-        "image": cb_helpers.get_env('CB_IMAGE_AWS', 'ami-052355af2a014bd2c'),
-        "vm_type": cb_helpers.get_env('CB_VM_TYPE_AWS', 't2.nano'),
-        "placement": cb_helpers.get_env('CB_PLACEMENT_AWS', 'us-east-1a'),
+        "image": env_or('CB_IMAGE_AWS', 'ami-052355af2a014bd2c'),
+        "vm_type": env_or('CB_VM_TYPE_AWS', 't2.nano'),
+        "placement": env_or('CB_PLACEMENT_AWS', 'us-east-1a'),
         "placement_cfg_key": "aws_zone_name"
     },
     'OpenStackCloudProvider': {
-        'image': cb_helpers.get_env('CB_IMAGE_OS',
-                                    'c66bdfa1-62b1-43be-8964-e9ce208ac6a5'),
-        "vm_type": cb_helpers.get_env('CB_VM_TYPE_OS', 'm1.tiny'),
-        "placement": cb_helpers.get_env('CB_PLACEMENT_OS', 'nova'),
+        'image': env_or('CB_IMAGE_OS',
+                        'c66bdfa1-62b1-43be-8964-e9ce208ac6a5'),
+        "vm_type": env_or('CB_VM_TYPE_OS', 'm1.tiny'),
+        "placement": env_or('CB_PLACEMENT_OS', 'nova'),
         "placement_cfg_key": "os_zone_name"
     },
     'GCPCloudProvider': {
-        'image': cb_helpers.get_env(
+        'image': env_or(
             'CB_IMAGE_GCP',
             'https://www.googleapis.com/compute/v1/projects/ubuntu-os-cloud/'
             'global/images/ubuntu-1804-bionic-v20200908'),
-        'vm_type': cb_helpers.get_env('CB_VM_TYPE_GCP', 'f1-micro'),
-        'placement': cb_helpers.get_env('GCP_ZONE_NAME', 'us-central1-a'),
+        'vm_type': env_or('CB_VM_TYPE_GCP', 'f1-micro'),
+        'placement': env_or('GCP_ZONE_NAME', 'us-central1-a'),
         "placement_cfg_key": "gcp_zone_name"
     },
     "AzureCloudProvider": {
         "image":
-            cb_helpers.get_env('CB_IMAGE_AZURE',
-                               'Canonical:0001-com-ubuntu-minimal-jammy:minimal-22_04-lts-gen2:latest'),
-        "vm_type": cb_helpers.get_env('CB_VM_TYPE_AZURE', 'Standard_DC1ds_v3'),
-        "placement": cb_helpers.get_env('CB_PLACEMENT_AZURE', 'eastus'),
+            env_or('CB_IMAGE_AZURE',
+                   'Canonical:0001-com-ubuntu-minimal-jammy:minimal-22_04-lts-gen2:latest'),
+        "vm_type": env_or('CB_VM_TYPE_AZURE', 'Standard_DC1ds_v3'),
+        "placement": env_or('CB_PLACEMENT_AZURE', 'eastus'),
         "placement_cfg_key": "azure_zone_name"
     }
 }