Add empty string in non-multi dictionary for empty lines (#1332)

Before 2.2.0, we only accepted input arguments in the form `key=value`.
To support the new `wwwauth[]` argument from Git, we added support for
these multi-value args `key[]=value`.

In changing from an dictionary of `string:string` to
`string:list<string>` we accidentally changed the behaviour of
dictionary parsing in the case that an empty valued (non-multi) argument
was provided. For example:

```text
username=\n
```

Previously this was returned as an empty string. Post 2.2.0 this became
`null`, causing an error in scenarios were before there was none.

One such scenario is with Windows Integrated Authentication (for example
when connecting to Azure DevOps Server/TFS instances), whereby we return
an empty username and password to Git to signal this auth mode. Git then
returns to us the empty username and password in a `store` call.

Update the handling in `ParseMultiLine` to restore the previous
behaviour for empty-valued arguments being mapped to the empty string.

Fixes #1331

---

**Bug reproduction steps:**

```console
% printf "url=https://example.com\nusername=\npassword=\n\n" | git credential approve
fatal: Missing 'username' input argument
```

src/shared/Core.Tests/StreamExtensionsTests.cs

@@ -254,6 +254,19 @@ public void StreamExtensions_ReadMultiDictionary_CaseInsensitive_ReturnsDictiona
             AssertMultiDictionary(new[] { "2" }, "a", output);
         }
 
+        [Fact]
+        public void StreamExtensions_ReadMultiDictionary_EmptyString_ReturnsKeyWithEmptyStringValue()
+        {
+            string input = "a=\n\n";
+
+            var output = ReadStringStream(input, StreamExtensions.ReadMultiDictionary);
+
+            Assert.NotNull(output);
+            Assert.Equal(1, output.Count);
+
+            AssertMultiDictionary(new[] { String.Empty,  }, "a", output);
+        }
+
         [Fact]
         public void StreamExtensions_ReadMultiDictionary_Spaces_ReturnsCorrectKeysAndValues()
         {

src/shared/Core/StreamExtensions.cs

@@ -248,14 +248,15 @@ private static void ParseMultiLine(IDictionary<string, IList<string>> dict, stri
                 // Only allow one value for non-multi/array entries ("key=value")
                 // and reset the array of a multi-entry if the value is empty ("key[]=<empty>")
                 bool emptyValue = string.IsNullOrEmpty(value);
+
                 if (!multi || emptyValue)
                 {
                     list.Clear();
+                }
 
-                    if (emptyValue)
-                    {
-                        return;
-                    }
+                if (multi && emptyValue)
+                {
+                    return;
                 }
 
                 list.Add(value);