url_prefix (and others): Cannot set empty environmnent variable default in config.yml

[SarahG-579462]

Description

It's not possible to set an environment variable such as url_prefix: ${PYGEOAPI_PREFIX:-} such that, if PYGEOAPI_PREFIX is undefined, no url_prefix is set.

I've tried ${PYGEOAPI_PREFIX:-""}, to no avail (url becomes /""/etc).

Steps to Reproduce

add url_prefix: ${PYGEOAPI_PREFIX:-} to the server api_rules in config.yml

Expected behavior
If PYGEOAPI_PREFIX is set, use this value. If it is not set, do not use a url_prefix

Screenshots/Tracebacks
If applicable, add screenshots to help explain your problem.

backend-1  |   File "/opt/conda/envs/backend/lib/python3.11/site-packages/pygeoapi/starlette_app.py", line 86, in <module>
backend-1  |     api_ = API(CONFIG, OPENAPI)
backend-1  |            ^^^^^^^^^^^^^^^^^^^^
backend-1  |   File "/opt/conda/envs/backend/lib/python3.11/site-packages/pygeoapi/api/__init__.py", line 658, in __init__
backend-1  |     self.base_url = get_base_url(self.config)
backend-1  |                     ^^^^^^^^^^^^^^^^^^^^^^^^^
backend-1  |   File "/opt/conda/envs/backend/lib/python3.11/site-packages/pygeoapi/util.py", line 216, in get_base_url
backend-1  |     return url_join(config['server']['url'], rules.get_url_prefix())
backend-1  |                                              ^^^^^^^^^^^^^^^^^^^^^^
backend-1  |   File "/opt/conda/envs/backend/lib/python3.11/site-packages/pygeoapi/models/config.py", line 95, in get_url_prefix
backend-1  |     prefix = self.url_prefix.format(
backend-1  |              ^^^^^^^^^^^^^^^^^^^^^^^
backend-1  | KeyError: 'PYGEOAPI_PREFIX'

Environment

• OS:
• Python version:
• pygeoapi version:

Additional context

It seems like an issue in the yaml parsing,

pygeoapi/pygeoapi/util.py

Line 171 in 6682b44

r'.*?\$\{(?P<varname>\w+)(:-(?P<default>[^}]+))?\}')

the regex r'.*?\$\{(?P<varname>\w+)(:-(?P<default>[^}]+))?\}' could be changed to r'.*?\$\{(?P<varname>\w+)(:-(?P<default>[^}]*))?\}'