CrontabSchedule.TryParse shouldn't throw for null input

This is to be consistent with TryParse methods on types in the .NET Framework that also don't throw for null input.

Author: atifaziz

NCrontab.Tests/CrontabScheduleTests.cs

@@ -42,6 +42,14 @@ public void CannotParseEmptyString()
             Assert.Throws<CrontabException>(() => CrontabSchedule.Parse(string.Empty));
         }
 
+        [Test]
+        public void TryParseNullString() =>
+            Assert.That(CrontabSchedule.TryParse(null), Is.Null);
+
+        [Test]
+        public void TryParseEmptyString() =>
+            Assert.That(CrontabSchedule.TryParse(string.Empty), Is.Null);
+
         [Test]
         public void AllTimeString()
         {
@@ -309,6 +317,10 @@ static void BadField(string expression, bool includingSeconds)
             {
                 IncludingSeconds = includingSeconds
             }));
+            Assert.That(CrontabSchedule.TryParse(expression, new ParseOptions
+            {
+                IncludingSeconds = includingSeconds
+            }), Is.Null);
         }
 
         [TestCase("bad * * * * *", false)]

NCrontab/AssemblyInfo.g.cs

@@ -1,8 +1,8 @@
 // This code was generated by a tool. Any changes made manually will be lost
 // the next time this code is regenerated.
-// Generated: Thu, 20 Oct 2016 06:08:37 GMT
-
+// Generated: Thu, 20 Oct 2016 06:50:00 GMT
+ 
 using System.Reflection;
-
+ 
 [assembly: AssemblyVersion("3.1.20120.0")]
-[assembly: AssemblyFileVersion("3.1.20120.608")]
+[assembly: AssemblyFileVersion("3.1.20120.650")]

NCrontab/CrontabSchedule.cs

@@ -99,10 +99,10 @@ public static CrontabSchedule Parse(string expression, ParseOptions options) =>
         public static CrontabSchedule TryParse(string expression) => TryParse(expression, null);
 
         public static CrontabSchedule TryParse(string expression, ParseOptions options) =>
-            TryParse(expression, options, v => v, _ => null);
+            TryParse(expression ?? string.Empty, options, v => v, _ => null);
 
         public static T TryParse<T>(string expression, Func<CrontabSchedule, T> valueSelector, Func<ExceptionProvider, T> errorSelector) =>
-            TryParse(expression, null, valueSelector, errorSelector);
+            TryParse(expression ?? string.Empty, null, valueSelector, errorSelector);
 
         public static T TryParse<T>(string expression, ParseOptions options, Func<CrontabSchedule, T> valueSelector, Func<ExceptionProvider, T> errorSelector)
         {