fix: raise descriptive error for empty/comments-only config files (#139)

[AmSach]

Fixes #139

What was broken

Starting the server with an empty (or comments-only) config file crashed
with an opaque Python error:

File "src/instrumentserver/config.py", line 50, in loadConfig
    if "instruments" not in rawConfig:
TypeError: argument of type 'NoneType' is not iterable

Root cause

ruamel.yaml's yaml.load() returns None when the file is empty
(0 bytes) or contains only YAML comments (no key/value pairs). The
loadConfig function in src/instrumentserver/config.py then ran
"instruments" not in rawConfig immediately on the result, and the
membership test on None raised TypeError.

The next line of the function already raises a clear AttributeError
("All configurations must be inside the 'instruments' field...") when
the key is missing — so a user who simply forgets to add the section
sees a helpful message, but a user with a totally empty or
comments-only file sees a raw Python TypeError instead. Both are
missing-key cases at heart, and they should be reported consistently.

Fix

Added a rawConfig is None guard right after yaml.load(). When
triggered, it raises an AttributeError whose message mirrors the
existing missing-key error and tells the user what to do next:

AttributeError: The config file '/path/to/serverConfig.yml' is empty.
It needs at least an 'instruments:' section.

Tests

Three new regression tests in test/pytest/test_config.py:

• test_empty_config_raises — empty file raises AttributeError
  whose message contains "empty".
• test_comments_only_config_raises — comments-only file raises the
  same AttributeError (yaml also returns None for these).
• test_empty_config_error_mentions_instruments — the error message
  guides the user toward the required 'instruments:' key.

All 21 tests in test_config.py pass.

Files changed

• src/instrumentserver/config.py — +11 lines, the None guard.
• test/pytest/test_config.py — +33 lines, three regression tests.