libcamera: yaml-parser: Differentiate between empty and empty string

When accessing a nonexistent key on a dict the YamlObject returns an
empty element. This element can happily be cast to a string which is
unexpected. For example the following statement:

yamlDict["nonexistent"].get<string>("default")

is expected to return "default" but actually returns "". Fix this by
introducing an empty type to distinguish between an empty YamlObject and
a YamlObject of type value containing an empty string. For completeness
add an isEmpty() function and an explicit cast to bool to be able to
test for that type.

Extend the tests accordingly.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>

test/meson.build

@@ -73,7 +73,7 @@ internal_tests = [
     {'name': 'timer-thread', 'sources': ['timer-thread.cpp']},
     {'name': 'unique-fd', 'sources': ['unique-fd.cpp']},
     {'name': 'utils', 'sources': ['utils.cpp']},
-    {'name': 'yaml-parser', 'sources': ['yaml-parser.cpp'], 'should_fail': true},
+    {'name': 'yaml-parser', 'sources': ['yaml-parser.cpp']},
 ]
 
 internal_non_parallel_tests = [