Fix DataRow error management

DataRow's RowError is supposed to be String.Empty to indicate
no errors, but is sometimes wrongly set to null, falsely indicating
errors. Fixed to be String.Empty.
Added test for DataRow error
Test uses SQLite in-memory DB for test

Also spaces to tabs
Correct code review comments on test
[corlib] Handle String::Format with escaped closing }. Fixes #22114
Fix NullReferenceException in Uri.Compare when only one of the Uris is null
Merge pull request #1172 from roji/datarow_null

Fix DataRow error management
Merge pull request #1221 from LogosBible/uri-compare

Fix NullReferenceException in Uri.Compare when only one of the Uris is null
Remove unused/undefined function.
Fix a warning in mono-conc-hashtable.
Update badge URL in README.md.
[MSBuild] Make sure assembly references with extensions are located correctly
Tests serialization of XmlNode field with attribute XmlAnyElement.
Covers #3211.
Test SerializationCodeGenerator.
XmlSerializerTests was mostly testing XmlSerializationReaderInterpreter and XmlSerializationWriterInterpreter.
XmlSerializerGeneratorTests forces the usage of SerializationCodeGenerator while running XmlSerializerTests.
[xbuild] Resolve output properties for skipped task to match task output when it's not skipped
Changed UsageCount and generationThreshold to zero based.
Added GetTypeFullName, fixes wrong nullable valuetype names.
Replaced incorrect bool.ToString() by GetLiteral(bool).
Added missing XmlTypeMapping.IsAny logic.
This complements commit 05444c7c1c570db730f17b6d15dd66d3728b6.
Fixed serialization of XmlNode field with attribute XmlAnyElement.
Fixes #3211.