null/Empty ikey from AIServiceOptions is overriding ikey from appsettings.json #989

If you are reporting bug/issue, please provide detailed Repro instructions.

Repro Steps

1. Enable AI:
   var aiOptions = new ApplicationInsightsServiceOptions();
   aiOptions.EnableHeartbeat = false;
   services.AddApplicationInsightsTelemetry(aiOptions);
2. Provide ikey in appsettings,json

Actual Behavior

ikey from appsettings.json is overwritten by the one in aiOptions which is empty.

Expected Behavior

Do not override aiOptionsIkey unless its not null/non empty.

Version Info

SDK Version (version of https://www.nuget.org/packages/Microsoft.ApplicationInsights.AspNetCore) :
.NET Core Version (TargetFramework in your .csproj file) :
How Application was onboarded with SDK(Installed Nugets/VisualStudio/Azure WebAppExtension) :
OS :
Hosting Info (IIS/Azure Web Apps/Running From Visual Studio etc) :