Cannot insert the value NULL into column '', table ''; column does not allow nulls. INSERT fails.

When trying to run sp_helpdb on one of the instances I got this error message

Server: Msg 515, Level 16, State 2, Procedure sp_helpdb, Line 53
Cannot insert the value NULL into column 'owner', table 'tempdb.dbo.#spdbdesc_ The statement has been terminated.

Root Cause:
sp_helpdb returns information about all of your databases on a given instance and can be very useful for different reasons - one can be to monitor the size of the databases. For the above error received - the resolution is pretty straight forward. As the error message states, the database needs to have an owner else it will try to insert a NULL value into the "Owner" column of the temporary table and it will fail to provide the output when the system stored procedure sp_helpdb gets executed.

Solution
And the resolution is pretty straight forward as well - find the databases which do not have an owner assigned and assign an owner to them. We can use sysdatabases or sys.databases (in SQL Server 2005 and 2008):

For SQL Server 2000:

select name as database_name, SUSER_SNAME(sid) as owner_name, crdate as create_date_time from master.dbo.sysdatabases

For SQL Server 2005:

select name as database_name, SUSER_SNAME(owner_sid) as owner_name, create_date as create_date_time from master.sys.databases

And then see which databases do not have an owner assigned to them. The ones that do not have an owner assigned to them can be assigned an owner using this command:

Use {DB Name Here}
GO
EXEC sp_changedbowner {put the owner name here}

Once that is done, execution of sp_helpdb will work perfectly fine.