fix: Some missed env variable changes from PR 2490 (#2538)

# What does this PR do?

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.

## Test Plan

The following permutations now all run (or attempt to run to the point
of complaining that they can't connect to chroma, vllm, etc) when before
they failed immediately on startup because of bad environment variable
substitions:

```
uv run llama stack run llama_stack/templates/starter/run.yaml
ENABLE_SQLITE_VEC=true uv run llama stack run llama_stack/templates/starter/run.yaml
ENABLE_PGVECTOR=true uv run llama stack run llama_stack/templates/starter/run.yaml
ENABLE_CHROMADB=true uv run llama stack run llama_stack/templates/starter/run.yaml

uv run llama stack run llama_stack/templates/remote-vllm/run.yaml
```
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->

Signed-off-by: Ben Browning <bbrownin@redhat.com>
Co-authored-by: raghotham <rsm@meta.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}",
             ),
         ),
     ]