Remove warning on DB/DW/DL emtpy lists

This warning was added in 781c90b940 as a
way of catching the following cases, which are most likely programmer
mistakes:

    DB 1,, 2
    DB 1, 2,

However, the warning was also triggered in the following case:

    DB

It can be used as a replacement of:

    DS 1

In this case, it shouldn't output a warning.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/asm/rgbasm.5

@@ -598,7 +598,7 @@ allocates a number of bytes.
 The content is undefined.
 This is the preferred method of allocationg space in a RAM section.
 You can, however, use
-.Ic DB , 
+.Ic DB ,
 .Ic DW
 and
 .Ic DL