Allow environment variables to be empty or whitespace. (Azure#2777)

Environment variable values may be empty or whitespace.
Docker allows these values for environment variable:
"key1="
"key2=   "

edge-agent/src/Microsoft.Azure.Devices.Edge.Agent.Edgelet/models/EnvVar.cs

@@ -8,7 +8,7 @@ public class EnvVar
         public EnvVar(string key, string value)
         {
             this.Key = Preconditions.CheckNonWhiteSpace(key, nameof(key));
-            this.Value = Preconditions.CheckNonWhiteSpace(value, nameof(value));
+            this.Value = value ?? string.Empty;
         }
 
         public string Key { get; }