handle empty files for diffing mode

fixes CNVS-38281

test plan
 - upload diffing file
 - upload empty diffing file with just headers
 - it should diff the files

Change-Id: I5d9c5dbf04c37e485de408ca038ba524461d0afe
Reviewed-on: https://gerrit.instructure.com/119610
Tested-by: Jenkins
QA-Review: Tucker McKnight <tmcknight@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>

gems/csv_diff/lib/csv_diff/diff.rb

@@ -36,8 +36,8 @@ module CsvDiff
       setup_output(current_csv.headers)
 
       @db.transaction do
-        insert("previous", row_previous, previous_csv, current_csv.headers)
+        insert("previous", row_previous, previous_csv, current_csv.headers) if row_previous
-        insert("current", row_current, current_csv, nil)
+        insert("current", row_current, current_csv, nil) if row_current
       end
 
       find_updates

gems/csv_diff/spec/csv_diff_spec.rb

@@ -76,6 +76,12 @@ pk1,col1,pk2,col2
 CSV
   end
 
+  def only_header_row
+    CSV.new(<<CSV, headers: true)
+pk1,col1,pk2,col2
+CSV
+  end
+
   context 'validation' do
     it 'rejects csvs without headers' do
       expect { subject.generate(csv1(headers: false), csv1) }.to raise_error(CsvDiff::Failure, /headers/)
@@ -90,6 +96,12 @@ CSV
     it 'requires at least one pk column to be present' do
       expect { subject.generate(missing_pk, missing_pk) }.to raise_error(CsvDiff::Failure, /primary key/)
     end
+
+    it 'handles empty files' do
+      cb = ->(row) { row['col2'] = 'deleted' }
+      output = subject.generate(csv1, only_header_row, deletes: cb)
+      expect(output.read).to eq "pk1,col1,pk2,col2\na,b,c,deleted\n1,2,3,deleted\n"
+    end
   end
 
   context 'creates and updates' do