stream: check for null instead of falsy in loops

Check for null in while loops. This is preparing the code for
the no-cond-assign ESLint rule.

PR-URL: https://github.com/nodejs/node/pull/41614
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/internal/streams/buffer_list.js

@@ -57,7 +57,7 @@ module.exports = class BufferList {
       return '';
     let p = this.head;
     let ret = '' + p.data;
-    while (p = p.next)
+    while ((p = p.next) !== null)
       ret += s + p.data;
     return ret;
   }
@@ -129,7 +129,7 @@ module.exports = class BufferList {
         break;
       }
       ++c;
-    } while (p = p.next);
+    } while ((p = p.next) !== null);
     this.length -= c;
     return ret;
   }
@@ -163,7 +163,7 @@ module.exports = class BufferList {
         break;
       }
       ++c;
-    } while (p = p.next);
+    } while ((p = p.next) !== null);
     this.length -= c;
     return ret;
   }