Allow null as separator in TailOptions, per docs (#37902)

To have no separator, `tail` docs specify to pass `null`, so it should be valid to pass. 

See: https://www.npmjs.com/package/tail#constructor-parameters

>separator: the line separator token (default: /[\r]{0,1}\n/ to handle linux/mac (9+)/windows). **Pass null if your file is binary there's no >line separator.**

types/tail/index.d.ts

@@ -6,7 +6,7 @@
 
 declare namespace Tail {
     interface TailOptions {
-        separator?: string | RegExp;
+        separator?: string | RegExp | null;
         fromBeginning?: boolean;
         fsWatchOptions?: Record<string, any>;
         follow?: boolean;