Fix minion crash when grains config option is empty

[BrianHa94]

What does this PR do?

Fixes a minion crash on startup caused by a grains: config option that isn't a mapping. YAML parses grains: with no value as None, and non-dict values like a string, number, or list are also possible — two separate code paths (apply_minion_config and salt.loader.grains(), which independently re-reads the raw config file off disk) assumed the option was always a dict. That crashed the minion with TypeError: 'NoneType' object is not iterable when building the __grains__ context wrapper.

Both locations now check isinstance(value, dict) and silently default the option to an empty dict ({}) for any non-dict value, not just None. The required shape of grains is documented in conf/minion and doc/ref/configuration/minion.rst rather than surfaced as a runtime warning.

What issues does this PR fix or reference?

Fixes #61321

Previous Behavior

A minion configured with a grains: config option that wasn't a mapping (e.g. grains: with no value, an empty string, or a scalar) would crash on startup with:

TypeError: 'NoneType' object is not iterable

raised from salt.utils.context.NamespacedDictWrapper.__init__ via the loader's __prep_mod_opts.

New Behavior

The minion starts normally. Any non-dict grains value is silently treated as {}. The required shape (grains must be a mapping; use grains: {} for an explicit empty section) is documented in the minion configuration reference and the example minion config file.

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Unit tests in tests/pytests/unit/test_config.py and tests/pytests/unit/loader/test_loader.py, plus an integration test in tests/pytests/integration/cli/test_salt_minion.py, all parametrized over a range of non-dict grains values (None, empty string, empty list, string, number, list).

Commits signed with GPG?

No

[dwoz]

not isinstance(opts["grains"], dict) -- catches empty and wrong-type in one guard. No valid grains is anything but a mapping. Drop the warning; document the shape.

conf/minion above #grains::

+#
+# The value of 'grains' must be a mapping. Use 'grains: {}' for an
+# explicit empty section. Any non-dict value is silently defaulted
+# to an empty dict.
 #grains:

doc/ref/configuration/minion.rst inside the grains entry, after the example:

+The value of ``grains`` must be a mapping. Use ``grains: {}`` for an
+explicit empty section. Any non-dict value is silently defaulted to
+an empty dict.
+
 .. conf_minion:: grains_blacklist

Tests: parametrize over ["", '""', "[]", "foo", "42", "[1, 2]"] (all defaulting to {}) in tests/unit/test_config.py + tests/pytests/unit/loader/test_loader.py; same for tests/pytests/integration/cli/test_salt_minion.py with [None, "", [], "foo", 42, [1, 2]], drop the log-file assertions and the import salt.utils.files.

[welcome]

Congratulations on your first PR being merged! 🎉