fix: Some missed env variable changes from PR 2490

Some templates were still using the old environment variable
substition syntax instead of the new one and were not getting
substituted properly.

Also, some places didn't handle the new None vs old empty string ("")
values that come from the conditional environment variable
substitution.

This gets the starter and remote-vllm distributions starting again,
and I tested various permutations of the starter as chroma and
pgvector needed some adjustments to their config classes to handle the
new possible `None` values. And, I had to tweak our `Provider` class
to also handle `None` values, for cases where we disable providers in
the starter config via environment variables.

This may not have caught everything that was missed, but I did grep
around quite a bit to try and find anything lingering.

Signed-off-by: Ben Browning <bbrownin@redhat.com>

llama_stack/templates/postgres-demo/postgres_demo.py

@@ -29,7 +29,7 @@ def get_distribution_template() -> DistributionTemplate:
             provider_id="vllm-inference",
             provider_type="remote::vllm",
             config=VLLMInferenceAdapterConfig.sample_run_config(
-                url="${env.VLLM_URL:http://localhost:8000/v1}",
+                url="${env.VLLM_URL:=http://localhost:8000/v1}",
             ),
         ),
     ]