Make #to_params return blank hashes if we have no data

spec/lib/api_entreprise/etablissement_adapter_spec.rb

@@ -79,6 +79,6 @@ describe ApiEntreprise::EtablissementAdapter do
         .to_return(body: 'Fake body', status: 404)
     end
 
-    it { expect(subject).to be_nil }
+    it { expect(subject).to eq({}) }
   end
 end

spec/lib/api_entreprise/rna_adapter_spec.rb

@@ -19,7 +19,7 @@ describe ApiEntreprise::RNAAdapter do
     let(:body) { '' }
     let(:status) { '404' }
 
-    it { is_expected.to eq(nil) }
+    it { is_expected.to eq({}) }
   end
 
   it { expect(subject).to be_an_instance_of(Hash) }