Change Nullable to default to enable in libraries src and ref projects

[eerhardt]

Now that 90% of our libraries are annotated for nulllable, it makes sense to have the default be enable. There are currently 26 libraries that aren't fully annotated yet:

Microsoft.Bcl.AsyncInterfaces
Microsoft.Extensions.DependencyInjection.Specification.Tests
Microsoft.Extensions.Hosting.Systemd
Microsoft.Extensions.Hosting.WindowsServices
Microsoft.NETCore.Platforms
Microsoft.XmlSerializer.Generator
System.CodeDom
System.ComponentModel.Composition.Registration
System.Composition
System.Composition.AttributedModel
System.Composition.Convention
System.Composition.Hosting
System.Composition.Runtime
System.Composition.TypedParts
System.Configuration.ConfigurationManager
System.Diagnostics.EventLog
System.Diagnostics.PerformanceCounter
System.DirectoryServices.AccountManagement
System.DirectoryServices.Protocols
System.IO.Ports
System.Management
System.Runtime.Caching
System.Security.Cryptography.Xml
System.Security.Permissions
System.ServiceModel.Syndication
System.Speech

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Issue Details

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Now that 90% of our libraries are annotated for nulllable, it makes sense to have the default be enable. There are currently 26 libraries that aren't fully annotated yet:

Microsoft.Bcl.AsyncInterfaces
Microsoft.Extensions.DependencyInjection.Specification.Tests
Microsoft.Extensions.Hosting.Systemd
Microsoft.Extensions.Hosting.WindowsServices
Microsoft.NETCore.Platforms
Microsoft.XmlSerializer.Generator
System.CodeDom
System.ComponentModel.Composition.Registration
System.Composition
System.Composition.AttributedModel
System.Composition.Convention
System.Composition.Hosting
System.Composition.Runtime
System.Composition.TypedParts
System.Configuration.ConfigurationManager
System.Diagnostics.EventLog
System.Diagnostics.PerformanceCounter
System.DirectoryServices.AccountManagement
System.DirectoryServices.Protocols
System.IO.Ports
System.Management
System.Runtime.Caching
System.Security.Cryptography.Xml
System.Security.Permissions
System.ServiceModel.Syndication
System.Speech

Author:	eerhardt
Assignees:	-
Labels:	area-Infrastructure-libraries
Milestone:	-

[ViktorHofer]

Do we only want to set this property if it isn't yet set? Why not set it unconditionally? What if another component like the SDK or Arcade sets that property in the future and with that overrides our setting?

[eerhardt]

I can't really imagine the SDK or Arcade changing the default from blank to anything other than enable in the future. But even if they did, they still could override our setting if they were imported after this file.

In general, I prefer not unconditionally setting properties in props/targets files. If someone explicitly gets imported before this props file, and sets the value, I don't think defaulting the value should override that.

[ViktorHofer]

I usually unconditionally overwrite properties / items if I want to set a (new) default regardless of what was previously set. In this case it sounds like we want to set our own default. That said, I don't have a strong opinion, so I'm fine leaving it as is if you prefer that.

[ViktorHofer]

LGTM except for the few questions. Thanks Eric

[eerhardt]

runtime-dev-innerloop (Build OSX x64 release Runtime_Debug) timing out is unrelated to this change.

Merging.