Clarify chained configuration TryGet semantics and apply review-thread fixes

src/libraries/Microsoft.Extensions.Configuration/src/ChainedConfigurationProvider.cs

@@ -38,11 +38,11 @@ public ChainedConfigurationProvider(ChainedConfigurationSource source)
         /// </summary>
         /// <param name="key">The key.</param>
         /// <param name="value">When this method returns, contains the value.</param>
-        /// <returns><see langword="true"/> if a value for the specified key was found, otherwise <see langword="false"/>.</returns>
+        /// <returns><see langword="true"/> if the chained configuration has a non-<see langword="null"/> value for the specified key, otherwise <see langword="false"/>.</returns>
         public bool TryGet(string key, out string? value)
         {
             value = _config[key];
-            return !string.IsNullOrEmpty(value);
+            return value is not null;
         }
 
         /// <summary>

src/libraries/Microsoft.Extensions.Configuration/tests/ChainedConfigurationProviderTests.cs

@@ -153,6 +153,38 @@ public void ChainedConfiguration_ReloadingOuterConfigurationRoot_RaisesSingleOut
             Assert.Equal(0, innerNotifications);
         }
 
+        [Fact]
+        public void ChainedConfiguration_TryGetReturnsTrueForEmptyStringValue()
+        {
+            IConfigurationProvider provider = new ChainedConfigurationSource
+            {
+                Configuration = new ConfigurationBuilder()
+                    .AddInMemoryCollection(new Dictionary<string, string> { { "Key", string.Empty } })
+                    .Build(),
+                ShouldDisposeConfiguration = false,
+            }
+            .Build(new ConfigurationBuilder());
+
+            Assert.True(provider.TryGet("Key", out string? value));
+            Assert.Equal(string.Empty, value);
+        }
+
+        [Fact]
+        public void ChainedConfiguration_TryGetReturnsFalseForMissingKey()
+        {
+            IConfigurationProvider provider = new ChainedConfigurationSource
+            {
+                Configuration = new ConfigurationBuilder()
+                    .AddInMemoryCollection(new Dictionary<string, string> { { "OtherKey", "inner-value" } })
+                    .Build(),
+                ShouldDisposeConfiguration = false,
+            }
+            .Build(new ConfigurationBuilder());
+
+            Assert.False(provider.TryGet("MissingKey", out string? value));
+            Assert.Null(value);
+        }
+
         private class TestConfigurationProvider : ConfigurationProvider
         {
             public TestConfigurationProvider(string key, string value)