[V4] Update credentials resolver to handle empty profile names in config

generator/.DevConfigs/fe2b0bd7-f45d-4cc5-ba88-0be51c08e324.json

@@ -0,0 +1,9 @@
+{
+    "core": {
+        "changeLogMessages": [
+            "Fix regression in V4 where the credentials resolver would attempt to fetch a profile even when its name was empty or null (https://github.com/aws/aws-sdk-net/issues/4028)"
+        ],
+        "type": "patch",
+        "updateMinimum": true
+    }
+}

sdk/src/Core/Amazon.Runtime/Credentials/DefaultAWSCredentialsIdentityResolver.cs

@@ -101,7 +101,7 @@ BaseIdentity IIdentityResolver.ResolveIdentity(IClientConfig clientConfig)
         public AWSCredentials ResolveIdentity(IClientConfig clientConfig)
         {
             var profile = clientConfig?.Profile;
-            if (profile != null)
+            if (!string.IsNullOrEmpty(profile?.Name))
             {
                 var source = new CredentialProfileStoreChain(profile.Location);
                 if (source.TryGetProfile(profile.Name, out CredentialProfile storedProfile))

sdk/test/UnitTests/Custom/Runtime/Credentials/DefaultCredentialsResolverTests.cs

@@ -13,9 +13,11 @@
  * permissions and limitations under the License.
  */
 
+using Amazon;
 using Amazon.Runtime;
 using Amazon.Runtime.CredentialManagement;
 using Amazon.Runtime.Credentials;
+using Amazon.S3;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using System;
 
@@ -88,5 +90,31 @@ public void CredentialsAreReevaluatedWhenProfileChanges()
             Environment.SetEnvironmentVariable(AWS_PROFILE_ENVIRONMENT_VARIABLE, "non-existent-profile");
             Assert.ThrowsException<ProfileNotFoundException>(() => identityResolver.ResolveIdentity(clientConfig: null));
         }
+
+        /// <summary>
+        /// Reported in https://github.com/aws/aws-sdk-net/issues/4028
+        /// <para />
+        /// If the name property in the "Profile" property is not set (which can happen when its value
+        /// is only set in a non-production environment), the resolver should not throw an exception and 
+        /// move to the next provider in the chain (which in this test will be the access / secret key pair
+        /// set in the environment variables).
+        /// </summary>
+        [DataTestMethod]
+        [DataRow("")]
+        [DataRow(null)]
+        public void CredentialsResolver_HandlesEmptyProfileNameInConfig(string profileName)
+        {
+            Environment.SetEnvironmentVariable(EnvironmentVariablesAWSCredentials.ENVIRONMENT_VARIABLE_ACCESSKEY, "foo");
+            Environment.SetEnvironmentVariable(EnvironmentVariablesAWSCredentials.ENVIRONMENT_VARIABLE_SECRETKEY, "bar");
+
+            var identityResolver = new DefaultAWSCredentialsIdentityResolver();
+            var resolvedIdentity = identityResolver.ResolveIdentity(new AmazonS3Config
+            {
+                Profile = new Profile(profileName)
+            });
+
+            Assert.IsNotNull(resolvedIdentity);
+            Assert.IsTrue(resolvedIdentity is EnvironmentVariablesAWSCredentials);
+        }
     }
 }