fix check for empty cells in rst template

used isspace before, which is wrong from empty strings

IPython/nbconvert/templates/rst.tpl

@@ -1,80 +1,80 b''
 {%- extends 'display_priority.tpl' -%}
 
 
 {% block in_prompt %}
 {% endblock in_prompt %}
 
 {% block output_prompt %}
 {% endblock output_prompt %}
 
 {% block input %}
-{%- if not cell.input.isspace() -%}
+{%- if cell.input.strip() -%}
 .. code:: python
 
 {{ cell.input | indent}}
 {%- endif -%}
 {% endblock input %}
 
 {% block pyerr %}
 ::
 
 {{ super() }}
 {% endblock pyerr %}
 
 {% block traceback_line %}
 {{ line | indent | strip_ansi }}
 {% endblock traceback_line %}
 
 {% block pyout %}
 {% block data_priority scoped %}
 {{ super() }}
 {% endblock %}
 {% endblock pyout %}
 
 {% block stream %}
 .. parsed-literal::
 
 {{ output.text | indent }}
 {% endblock stream %}
 
 {% block data_svg %}
 .. image:: {{ output.svg_filename|urlencode }}
 {% endblock data_svg %}
 
 {% block data_png %}
 .. image:: {{ output.png_filename|urlencode }}
 {% endblock data_png %}
 
 {% block data_jpg %}
 .. image:: {{ output.jpeg_filename|urlencode }}
 {% endblock data_jpg %}
 
 {% block data_latex %}
 .. math::
 
 {{ output.latex | strip_dollars | indent }}
 {% endblock data_latex %}
 
 {% block data_text scoped %}
 .. parsed-literal::
 
 {{ output.text | indent }}
 {% endblock data_text %}
 
 {% block data_html scoped %}
 .. raw:: html
 
 {{ output.html | indent }}
 {% endblock data_html %}
 
 {% block markdowncell scoped %}
 {{ cell.source | markdown2rst }}
 {% endblock markdowncell %}
 
 {% block headingcell scoped %}
 {{ ("#" * cell.level + cell.source) | replace('\n', ' ') | markdown2rst }}
 {% endblock headingcell %}
 
 {% block unknowncell scoped %}
 unknown type  {{cell.type}}
 {% endblock unknowncell %}