Add defensive checks to deps.json and runtimeconfig.json predictions

[dfederm]

Based on additional testing I found that *.runtimeconfig.dev.json files are no longer produced as of .NET SDK 6:

  <PropertyGroup Condition="'$(GenerateRuntimeConfigDevFile)' == ''">
    <GenerateRuntimeConfigDevFile>true</GenerateRuntimeConfigDevFile>
    <!-- Post-net6.0, stop generating *.runtimeconfig.dev.json files to reduce probing paths. -->
    <!-- https://github.com/dotnet/sdk/issues/16818 -->
    <GenerateRuntimeConfigDevFile Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '6.0'))">false</GenerateRuntimeConfigDevFile>
  </PropertyGroup>

I added defensive checks everywhere though just in case anything happens to be unexpectedly missing.