[Xamarin.Android.Build.Tasks] guard AutoImport.props against empty values

[jonathanpeppers]

There is an email thread about ASP.NET projects in VS -- and something is potentially slowing down builds when optional workloads are installed.

One concern is our AutoImport.props, take for instance the following foo.targets file:

<Project>
  <ItemGroup>
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.xml" />
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.axml" />
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.png" />
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.jpg" />
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.gif" />
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.webp" />
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\font\*.ttf" />
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\font\*.otf" />
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\font\*.ttc" />
    <AndroidResource Include="$(MonoAndroidResourcePrefix)\raw\*" Exclude="$(MonoAndroidResourcePrefix)\raw\.*" />
    <AndroidAsset Include="$(MonoAndroidAssetsPrefix)\**\*" Exclude="$(MonoAndroidAssetsPrefix)\**\.*\**" />
  </ItemGroup>
  <Target Name="Build" />
</Project>

In this case $(MonoAndroidResourcePrefix) and $(MonoAndroidAssetsPrefix) will be blank, and we get:

msbuild foo.targets -bl
...
  MSBUILD : warning MSB5029: The value "\**\.*\**" of the "Exclude" attribute in element <ItemGroup> in file "foo.targets (13,61)" is a wildcard that results in enumerating all files on the drive, which was likely not intended. Check that referenced properties are always defined.
1 Warning(s)
0 Error(s)
Time Elapsed 00:02:45.14

I'm not sure this is causing an actual problem, but I think it's a good idea to add some safety here.

I will make a similar change in Xamarin.Legacy.Sdk.

[jonpryor]

Similar to above, but…

Why support $(MonoAndroidAssetsPrefix) at all anymore? Consider @(TransformFile), @(AndroidLibrary), @(AndroidJavaSource), etc., below: none of those use $(MonoAndroidAssetsPrefix), and thus aren't susceptible to pulling in C:\**\*.

Alternatively, perhaps we could require/force $(MonoAndroidAssetsPrefix) to end with "directory-separator-char", which would allow us to change these includes to e.g.:

<AndroidResource Include="$(MonoAndroidAssetsPrefix)**\*" />

which would also prevent us from trying to traverse C:\**\*.

[jonathanpeppers]

After discussion and thinking about AutoImports.props MSBuild ordering, we decided to just format my code better...

Spec if needed: https://github.com/dotnet/designs/blob/main/accepted/2020/workloads/workload-resolvers.md#workload-props-files