Don't return empty hashes

This happened when rebasing and converting to unavailable currencies. For instance, the following was returning empty hashes for dates prior to the release of the new Turkish Lira:

https://frankfurter.app/1999-01-01..?from=USD&to=TRY

.rubocop.yml

@@ -4,5 +4,7 @@ Documentation:
   Enabled: false
 Metrics/BlockLength:
   ExcludedMethods: ['describe', 'helpers']
+Metrics/AbcSize:
+  Max: 20.45
 Metrics/MethodLength:
-  Max: 11
+  Max: 13

lib/quote/base.rb

@@ -64,19 +64,18 @@ module Quote
 
     def rebase_rates
       result.each do |date, rates|
-        add_euro(rates)
-
+        rates['EUR'] = amount if symbols.nil? || symbols.include?('EUR')
         divisor = rates.delete(base)
-        result[date] = rates.sort
-                            .map! do |iso_code, rate|
-                              [iso_code, round(amount * rate / divisor)]
-                            end
-                            .to_h
+        if rates.empty?
+          result.delete(date)
+        else
+          result[date] = rates.sort
+                              .map! do |iso_code, rate|
+                                [iso_code, round(amount * rate / divisor)]
+                              end
+                              .to_h
+        end
       end
     end
-
-    def add_euro(rates)
-      rates['EUR'] = amount if symbols.nil? || symbols.include?('EUR')
-    end
   end
 end

spec/quote/base_spec.rb

@@ -34,11 +34,9 @@ module Quote
     end
 
     describe 'when given data' do
-      let(:klass) do
-        Class.new(Base) do
-          def fetch_data
-            []
-          end
+      before do
+        def quote.fetch_data
+          []
         end
       end
 
@@ -51,5 +49,26 @@ module Quote
         refute quote.perform
       end
     end
+
+    describe 'when rebasing and converting to an unavailable currency' do
+      let(:date) do
+        Date.today
+      end
+
+      let(:quote) do
+        klass.new(date: date, base: 'USD', symbols: ['FOO'])
+      end
+
+      before do
+        def quote.fetch_data
+          [{ date: date, iso_code: 'USD', rate: 1.2 }]
+        end
+      end
+
+      it 'finds nothing' do
+        quote.perform
+        quote.not_found?.must_equal true
+      end
+    end
   end
 end