Raise error on empty paged responses

lib/vbms/service/paged_documents.rb

@@ -11,6 +11,9 @@ def call(file_number:)
         req = next_request(file_number, 0)
         first_page = client.send_request(req)
 
+        # interpret a first page with no sections (and no doc count) as equivalent to zero pages.
+        raise ClientError.new("No sections found in first page") if first_page.empty?
+
         # response will always be an array. get pagination from the first section.
         (documents << first_page.map { |section| section[:documents] }).flatten!
         pagination = first_page.first[:paging]

spec/service/paged_documents_spec.rb

@@ -81,5 +81,12 @@ def next_response
         expect(r[:paging][:@total_result_count]).to eq total_docs
       end
     end
+
+    context "when the first page contains no sections" do
+      it "raises a ClientError" do
+        allow(client).to receive(:send_request).and_return []
+        expect { subject.call(file_number: file_number) }.to raise_error(VBMS::ClientError)
+      end
+    end
   end
 end