refactor: remove configuration section check

Author: cnblogs-dudu

src/Enyim.Caching/EnyimMemcachedServiceCollectionExtensions.cs

@@ -67,11 +67,6 @@ public static IServiceCollection AddEnyimMemcached(
                 throw new ArgumentNullException(nameof(configurationSection));
             }
 
-            if (!configurationSection.Exists())
-            {
-                throw new ArgumentNullException($"{configurationSection.Key} in appsettings.json");
-            }
-
             return services.AddEnyimMemcachedInternal(
                 s => s.Configure<MemcachedClientOptions>(configurationSection), asDistributedCache);
         }
@@ -93,10 +88,6 @@ public static IServiceCollection AddEnyimMemcached(
             }
 
             var section = configuration.GetSection(sectionKey);
-            if (!section.Exists())
-            {
-                throw new ArgumentNullException($"{sectionKey} in appsettings.json");
-            }
 
             return services.AddEnyimMemcachedInternal(
                 s => s.Configure<MemcachedClientOptions>(section), asDistributedCache);
@@ -157,12 +148,6 @@ public static IServiceCollection AddEnyimMemcached<T>(
                 throw new ArgumentNullException(nameof(configuration));
             }
 
-            var section = configuration.GetSection(sectionKey);
-            if (!section.Exists())
-            {
-                throw new ArgumentNullException($"{sectionKey} in appsettings.json");
-            }
-
             return services.AddEnyimMemcached<T>(
                s => s.Configure<MemcachedClientOptions>(configuration.GetSection(sectionKey)));
         }