Fix compile error when preloading optional associations (#925)

spec/avram/preloading/preloading_belongs_to_spec.cr

@@ -209,5 +209,15 @@ describe "Preloading belongs_to associations" do
         comments[1].post.title.should eq("THIS IS CHANGED")
       end
     end
+
+    it "works for optional associations" do
+      with_lazy_load(enabled: false) do
+        business = BusinessFactory.new.create
+        email_address = EmailAddressFactory.create &.business_id(business.id)
+        email_address = EmailAddressQuery.preload_business(email_address)
+
+        email_address.business.try(&.id).should eq(business.id)
+      end
+    end
   end
 end

src/avram/associations/belongs_to.cr

@@ -61,7 +61,8 @@ module Avram::Associations::BelongsTo
         return record if record._{{ assoc_name }}_preloaded? && !force
 
         new_record = record.dup
-        assoc = preload_query.id(record.{{ foreign_key }}).first?
+        assoc = record.{{ foreign_key }}.try { |id| preload_query.id(id).first? }
+
         new_record.__set_preloaded_{{ assoc_name }}(assoc)
         new_record
       end