fixed missing checks for credentials

Author: John Campion Jr

test/FluentEmail.Core.Tests/ThirdParty/AzureEmailSenderTests.cs

@@ -21,7 +21,7 @@ public class AzureEmailSenderTests
 
     public AzureEmailSenderTests()
     {
-        Sender = new AzureEmailSender(_connectionString);
+        if (!string.IsNullOrEmpty(_connectionString)) Sender = new AzureEmailSender(_connectionString);
     }
 
     [Fact]

test/FluentEmail.Core.Tests/ThirdParty/PostmarkSenderTests.cs

@@ -277,27 +277,31 @@ public async Task SimpleMailFromCodeWithHeaders()
     [Fact]
     public void SenderNullServerToken()
     {
+        Assert.SkipWhen(string.IsNullOrEmpty(_apiKey), "No Postmark Credentials");
         Func<PostmarkSender> fn = () => new PostmarkSender((string)null!);
         fn.Should().Throw<ArgumentNullException>();
     }
 
     [Fact]
     public void OptionsNullServerToken()
     {
+        Assert.SkipWhen(string.IsNullOrEmpty(_apiKey), "No Postmark Credentials");
         Func<PostmarkSenderOptions> fn = () => new PostmarkSenderOptions(null!);
         fn.Should().Throw<ArgumentNullException>();
     }
 
     [Fact]
     public void NullOptions()
     {
+        Assert.SkipWhen(string.IsNullOrEmpty(_apiKey), "No Postmark Credentials");
         Func<PostmarkSender> fn = () => new PostmarkSender((PostmarkSenderOptions)null!);
         fn.Should().Throw<ArgumentNullException>();
     }
 
     [Fact]
     public void SendNull()
     {
+        Assert.SkipWhen(string.IsNullOrEmpty(_apiKey), "No Postmark Credentials");
         var sender = new PostmarkSender(_apiKey);
         Func<Task> fn = async () => await sender.SendAsync(null!).ConfigureAwait(false);
         fn.Should().ThrowAsync<ArgumentNullException>();