Is there some default password for a new user in Linux?

Authentication can be handled in many different ways in Linux. Password authentication via /etc/passwd and /etc/shadow is the usual default. There is no default password.

A user is not required to have a password. In a typical setup a user without a password will be unable to authenticate with the use of a password. This is common for system users which are used to run daemons, but are not intended to be used directly by a human.

You can configure Linux to allow login to the desktop automatically, or allow login without a password. Authentication is done via PAM, which is highly configurable. The Arch wiki offers the following PAM configuration for login without a password:

If you want to bypass the password prompt in GDM then simply add the following line on the first line of /etc/pam.d/gdm-password:

auth sufficient pam_succeed_if.so user ingroup nopasswdlogin 

Then, add the group nopasswdlogin to your system. See Groups for group descriptions and group management commands. Now, add your user to the nopasswdlogin group and you will only have to click on your username to login.

To answer the literal question: no, there is no default password. Usually by default an account will have an "invalid" password, that is, a password hash that will not be matched by any password at all. In order to be able to log in, a password must be explicitly specified, e.g. by running passwd for the account in question.

However, you can configure the logon manager to automatically log in as a specific user, either immediately or after a delay, if you wish to do so. That is not normally done, but the feature exists.

No, there is not. For every single user you create in the GUI you have to set manually the password. Blank passwords or default standard password are obviously considered as a high security risk.

But

You have the command newusers witch can help you create one or many users using the default password you want in plain text mode. usage as root : newusers file where file is a text file with the folowing structure for each line : pw_name:pw_passwd:pw_uid:pw_gid:pw_gecos:pw_dir:pw_shell

You can do it also in one command line. Example :

echo chinmay:clearpassword::::/home/maleki:/bin/bash | sudo newusers