Enable IDE0030 (Use coalesce expression) (#70948)

Author: stephentoub

eng/CodeAnalysis.src.globalconfig

@@ -1386,7 +1386,7 @@ dotnet_diagnostic.IDE0028.severity = suggestion
 dotnet_diagnostic.IDE0029.severity = warning
 
 # IDE0030: Use coalesce expression
-dotnet_diagnostic.IDE0030.severity = suggestion
+dotnet_diagnostic.IDE0030.severity = warning
 
 # IDE0031: Use null propagation
 dotnet_diagnostic.IDE0031.severity = silent

src/libraries/System.Net.Requests/src/System/Net/HttpWebResponse.cs

@@ -80,8 +80,7 @@ public override long ContentLength
             get
             {
                 CheckDisposed();
-                long? length = _httpResponseMessage.Content?.Headers.ContentLength;
-                return length.HasValue ? length.Value : -1;
+                return _httpResponseMessage.Content?.Headers.ContentLength ?? -1;
             }
         }