Allow for the absence of codec with decoding ocf

[gfikes]

In regards to avro.codec when decoding ocf, The Apache Avro docs state the following:

Implementations are required to support the following codecs: "null" and "deflate". If codec is absent, it is assumed to be "null".

When trying to decode an ocf file when codec is absent, I get the following error:

** (CaseClauseError) no case clause matching: false
    (erlavro) src/avro_ocf.erl:261: :avro_ocf.get_codec/1
    (erlavro) src/avro_ocf.erl:75: :avro_ocf.decode_binary/1

This change simply sets the codec to null when it is absent from the file.

[coveralls]

Pull Request Test Coverage Report for Build 541

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0006%) to 98.992%

---

Totals
Change from base Build 538:	0.0006%
Covered Lines:	1669
Relevant Lines:	1686

---

💛 - Coveralls

[zmstone]

@gfikes thanks! the new lines are not covered by tests, could you add a test case for it?

[zmstone]

and please bump version number to 2.7.1 in .app.src

[zmstone]

tagged 2.7.1, also published to hex. thank you @gfikes