[Xamarin.Android.Build.Tasks] guard AutoImport.props against empty values (#7837)

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`.  Consider this example
`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 example `$(MonoAndroidResourcePrefix)` and
`$(MonoAndroidAssetsPrefix)` will be blank, so when we build:

	> 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 if this is causing an actual problem, but I think it's a
good idea to add some safety here.

Update `AutoImport.props` to avoid including item groups when
`$(MonoAndroidResourcePrefix)` and `$(MonoAndroidAssetsPrefix)` are
not set/empty.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/Sdk/AutoImport.props

@@ -22,7 +22,9 @@ https://github.com/dotnet/designs/blob/4703666296f5e59964961464c25807c727282cae/
     <Using Include="Android.OS.Bundle" Alias="Bundle" Platform="Android" />
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(EnableDefaultAndroidItems)' == 'true' and $([MSBuild]::VersionEquals($(TargetFrameworkVersion), '8.0')) ">
+  <ItemGroup Condition=" '$(MonoAndroidResourcePrefix)' != '' and
+      '$(EnableDefaultAndroidItems)' == 'true' and
+      $([MSBuild]::VersionEquals($(TargetFrameworkVersion), '8.0')) ">
     <!-- Default Resource file inclusion -->
     <!-- https://developer.android.com/guide/topics/resources/providing-resources -->
     <AndroidResource Include="$(MonoAndroidResourcePrefix)\*\*.xml" />
@@ -35,8 +37,17 @@ https://github.com/dotnet/designs/blob/4703666296f5e59964961464c25807c727282cae/
     <AndroidResource Include="$(MonoAndroidResourcePrefix)\font\*.otf" />
     <AndroidResource Include="$(MonoAndroidResourcePrefix)\font\*.ttc" />
     <AndroidResource Include="$(MonoAndroidResourcePrefix)\raw\*" Exclude="$(MonoAndroidResourcePrefix)\raw\.*" />
+  </ItemGroup>
+
+  <ItemGroup Condition=" '$(MonoAndroidAssetsPrefix)' != '' and
+      '$(EnableDefaultAndroidItems)' == 'true' and
+      $([MSBuild]::VersionEquals($(TargetFrameworkVersion), '8.0')) ">
     <!-- Default Asset file inclusion -->
     <AndroidAsset Include="$(MonoAndroidAssetsPrefix)\**\*" Exclude="$(MonoAndroidAssetsPrefix)\**\.*\**" />
+  </ItemGroup>
+
+  <ItemGroup Condition=" '$(EnableDefaultAndroidItems)' == 'true'
+      and $([MSBuild]::VersionEquals($(TargetFrameworkVersion), '8.0')) ">
     <!-- Default XPath transforms for bindings -->
     <TransformFile Include="Transforms*.xml" />
     <TransformFile Include="Transforms\**\*.xml" />