How can I start the IDE with predefined username and database name. Password must be empty. Which command line parameters should be?

You can use the userid parameter. For example:

plsqldev.exe userid=scott@chicago

When I pass parameter as plsqldev.exe userid=scott@chicago, I get the error ORA-01005: null password given; logon denied.

This is very annoying.

There is currently no option to set a default username and password.

Hi, is it possible to make a connection from command line as sysdba?

Yes, but you will have to enclose the parameter value in double quotes, so that the spaces are parsed correctly. For example:

plsqldev.exe userid="sys/manager@chicago as sysdba"

To log in as a specific user, you could set up the user in Connections with no password, and check "Connect on startup". Then starting PL/SQL Developer prompts for login credentials on startup with user and database service pre-filled. (Not exactly what you are after, but might be close.)