Add nil check to cast_polymorphic_embeds_many

[clinejj]

Add a nil check for changes introduced in 9bc13e2 that supported maintaining IDs

Fixes #105

[mathieuprog]

Can we just do:

list_data_for_field = Map.fetch!(changeset.data, field) || []

Cause the variable says list_ so we can expect a list.

[clinejj]

@mathieuprog yep, that's a lot cleaner - updated with that change

[coveralls]

Pull Request Test Coverage Report for Build cef782eca8b2df3a9cabc5a15e3834c28019d9cd-PR-106

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.008%

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Totals
Change from base Build c57a335e2918b4d5e97af61d030f92dc44255fae:	0.0%
Covered Lines:	298
Relevant Lines:	359

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build cef782eca8b2df3a9cabc5a15e3834c28019d9cd-PR-106

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.008%

---

Totals
Change from base Build c57a335e2918b4d5e97af61d030f92dc44255fae:	0.0%
Covered Lines:	298
Relevant Lines:	359

---

💛 - Coveralls