reports: fix error when report is empty.

Fix an exception when a report is empty, for example when performing a
tag search that returns no results:

* https://betabooru.donmai.us/reports/posts?search[group]=uploader&search[tags]=does_not_exist

app/components/time_series_component.rb

@@ -18,7 +18,7 @@ class TimeSeriesComponent < ApplicationComponent
 
   def chart_height
     if x_axis != "date"
-      dataframe[x_axis].size * 30
+      dataframe[x_axis]&.size.to_i * 30
     end
   end
 

app/components/time_series_component/time_series_component.html.erb

@@ -6,7 +6,9 @@
   <% end %>
 </div>
 
-<% if mode == :chart && x_axis.present? %>
+<% if dataframe.empty? %>
+  <p>No results found. Try modifying your search or expanding your time range.</p>
+<% elsif mode == :chart && x_axis.present? %>
   <div class="line-chart" style="width: 100%; height: max(90vh, <%= chart_height.to_i %>px);"></div>
 
   <script type="text/javascript">