Also handle case where prompt number is undefined

IPython/nbconvert/templates/html/basic.tpl

@@ -23,7 +23,11 b''
 
 {% block in_prompt -%}
 <div class="prompt input_prompt">
+{%- if cell.prompt_number is defined -%}
 In&nbsp;[{{ cell.prompt_number|replace(None, "&nbsp;") }}]:
+{%- else -%}
+In&nbsp;[&nbsp;]:
+{%- endif -%}
 </div>
 {%- endblock in_prompt %}
 
@@ -51,8 +55,12 b' In&nbsp;[{{ cell.prompt_number|replace(None, "&nbsp;") }}]:'
 <div class="output_area">
 {%- if output.output_type == 'pyout' -%}
     <div class="prompt output_prompt">
+{%- if cell.prompt_number is defined -%}
     Out[{{ cell.prompt_number|replace(None, "&nbsp;") }}]:
 {%- else -%}
+    Out[&nbsp;]:
+{%- endif -%}
+{%- else -%}
     <div class="prompt">
 {%- endif -%}
     </div>