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
[monodoc] Changes to support new-style iOS documentation:

- Fixed bug in creating CREF strings.
- Refactored XML file processing for customization during new style iOS assembly. This was done through the new file source interface, which lets you use dependency injection to customize the process.
- Added the ability to rewrite CREF references when assembling docs that use native types. Also, the ability to handle dropped namespaces.
[mdoc] Changes to support new-style iOS API:

- the dropns flag now lets you define a namespace that is dropped in a new version of the assembly during update.
- the ntypes flag lets you tell the update command that certain methods are now using native types, and as such will attempt to match with old types in order to preserve documentation.
- the preserve flag will avoid removing members. This is to avoid dropping documentation for members that changed from one version of the framework to another new-style version (ie. iOS).
[TimeZoneInfo] Fix issue with some floating DST transition times creating invalid DateTime values, and implement ToSerializedString and FromSerializedString.
This change is released under the MIT license.
Add test for Regex.Match with anchors on a substring.
Regex fixes for substring parsing.
Miscellaneous changes per review
Add test for Regex RTL substring matching.
Correct code review comments on test
Merge pull request #1192 from LogosBible/regex-fixes-21803

Regex substring fix
[mcs] auto-property initializer can be array initializer
[mcs] Any type parameter is allowed for null propagating operator
[mcs] nameof operator
[corlib] Remove heavyweight EndsWith from 1bff9d890f95e72e4145d48980a05908c87b53ce
[System.Net.Http] Handle quoted filename value. Fixes #21960
Re-enable valid tests
[runtime] Fix logic when allocating code region if the first try at a fixed address fails.
[System] Implemented .NET 4.5 overloads for GZipStream and DeflateStream.

Fixes xambug #21982.