Invoice Templates may show "- 0" as amount paid

[sbts]

While testing tonight I noticed that the fairly "unmodified" sales invoice template prints the amount paid with a negative sign IF the amount is zero

[ehuelsmann]

I'd benefit from a thorough test case here.

[sbts]

• Create any invoice on a DB with the default sales invoice template
• Print the invoice

The "Amount Paid" field would currently show -0 instead of 0