Add config getter defaults as in Akka.Configuration (#178)

* Add config getter defaults as in Akka.Configuration

* Updated tests

Author: IgorFedchenko

src/Hocon.Tests/HoconTests.cs

@@ -420,8 +420,9 @@ public void CanSetDefaultValuesWhenGettingData()
             ;
 
             emptyConfig.GetStringList(missingKey, new List<string>()).Should().Equal(new List<string>());
-            emptyConfig.Invoking(c => c.GetStringList(missingKey)).Should().Throw<HoconValueException>().Which
-                .InnerException.Should().BeOfType<HoconParserException>();
+            
+            emptyConfig.Invoking(c => c.GetStringList(missingKey)).Should().NotThrow("String list is an exception of the rule")
+                .And.Subject().Should().BeEquivalentTo(new List<string>());
             ;
         }
 

src/Hocon/HoconRoot.cs

@@ -662,8 +662,7 @@ public virtual IList<byte> GetByteList(HoconPath path, IList<byte> @default = nu
         public virtual IList<string> GetStringList(string path)
         {
             return WrapWithValueException(path,
-                () => GetStringList(HoconPath.Parse(path)) ??
-                      throw new HoconParserException($"Hocon path {path} does not exist."));
+                () => GetStringList(HoconPath.Parse(path)) ?? new List<string>());
         }
 
         /// <summary>