Fix null-pointer exception in credential parsing (#44)

Author: Helcaraxan

apis/platform_common/Credential.cs

@@ -107,7 +107,7 @@ public PlatformRefreshTokenCredential(IAuthorizationCodeFlow flow, string userId
         private static PlatformRefreshTokenCredential GetTokenCredentialAutomatically()
         {
             var serverRefreshToken = Environment.GetEnvironmentVariable("IMPROBABLE_PLATFORM_REFRESH_TOKEN");
-            if (serverRefreshToken != "")
+            if (!string.IsNullOrEmpty(serverRefreshToken))
             {
                 var clientSecrets = new ClientSecrets
                 {
@@ -127,7 +127,7 @@ private static PlatformRefreshTokenCredential GetTokenCredentialAutomatically()
             };
 
             var tokenFile = possibleTokenFiles.FirstOrDefault(File.Exists);
-            if (tokenFile != null)
+            if (!string.IsNullOrEmpty(tokenFile))
             {
                 try
                 {
@@ -144,4 +144,4 @@ private static PlatformRefreshTokenCredential GetTokenCredentialAutomatically()
             throw new NoRefreshTokenFoundException(RefreshTokenNotFoundMessage);
         }
     }
-}
+}