See encoding of the same data with schema1 and schema1 with an additional "optional" field. Note that the absence of the "additional " field is represented by an empty byte.

[nuriaruiz@nurieta][/workplace/analytics/refinery/source/refinery-camus/src/test/resources/avro]$ more ./hexdump.original
00000000 a4 13 08 74 6f 74 6f 08 62 6c 61 68 0a 31 32 33 |...toto.blah.123|
00000010 34 35 06 31 32 33 0c 31 32 33 34 35 36 06 0a 74 |45.123.123456..t|
00000020 65 73 74 31 0a 74 65 73 74 32 0a 74 65 73 74 33 |est1.test2.test3|
00000030 00 00 |..|
00000032
[nuriaruiz@nurieta][/workplace/analytics/refinery/source/refinery-camus/src/test/resources/avro]$ more ./hexdump.new
00000000 a4 13 08 74 6f 74 6f 08 62 6c 61 68 0a 31 32 33 |...toto.blah.123|
00000010 34 35 06 31 32 33 0c 31 32 33 34 35 36 06 0a 74 |45.123.123456..t|
00000020 65 73 74 31 0a 74 65 73 74 32 0a 74 65 73 74 33 |est1.test2.test3|
00000030 00 00 00 |...|
00000033