Address Analyzer Warnings (#2078)

* Address potential errors

* Add tests

* Add clarity

* Run formatting

test/Core.Test/Models/Business/BillingInfo.cs

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+using Bit.Core.Models.Business;
+using Xunit;
+
+namespace Bit.Core.Test.Models.Business
+{
+    public class BillingInfoTests
+    {
+        [Fact]
+        public void BillingInvoice_Amount_ShouldComeFrom_InvoiceTotal()
+        {
+            var invoice = new Stripe.Invoice
+            {
+                AmountDue = 1000,
+                Total = 2000,
+            };
+
+            var billingInvoice = new BillingInfo.BillingInvoice(invoice);
+
+            // Should have been set from Total
+            Assert.Equal(20M, billingInvoice.Amount);
+        }
+    }
+}

test/Core.Test/Services/AppleIapServiceTests.cs

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+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Bit.Core.Services;
+using Bit.Test.Common.AutoFixture;
+using Bit.Test.Common.AutoFixture.Attributes;
+using Microsoft.Extensions.Logging;
+using NSubstitute;
+using NSubstitute.Core;
+using Xunit;
+
+namespace Bit.Core.Test.Services
+{
+    [SutProviderCustomize]
+    public class AppleIapServiceTests
+    {
+        [Theory, BitAutoData]
+        public async Task GetReceiptStatusAsync_MoreThanFourAttempts_Throws(SutProvider<AppleIapService> sutProvider)
+        {
+            var result = await sutProvider.Sut.GetReceiptStatusAsync("test", false, 5, null);
+            Assert.Null(result);
+
+            var errorLog = sutProvider.GetDependency<ILogger<AppleIapService>>()
+                .ReceivedCalls()
+                .SingleOrDefault(LogOneWarning);
+
+            Assert.True(errorLog != null, "Must contain one error log of warning level containing 'null'");
+
+            static bool LogOneWarning(ICall call)
+            {
+                if (call.GetMethodInfo().Name != "Log")
+                {
+                    return false;
+                }
+
+                var args = call.GetArguments();
+                var logLevel = (LogLevel)args[0];
+                var exception = (Exception)args[3];
+
+                return logLevel == LogLevel.Warning && exception.Message.Contains("null");
+            }
+        }
+    }
+}