don't assume outputs have metadata

in nbconvert

some old notebooks do not have this key defined

IPython/nbconvert/templates/html/basic.tpl

@@ -120,10 +120,10 b' unknown type {{ cell.type }}'
 
 {% block data_png %}
 <img src="data:image/png;base64,{{ output.png }}"
-{%- if 'width' in output.metadata.get('png', {}) %}
+{%- if 'metadata' in output and 'width' in output.metadata.get('png', {}) %}
 width={{output.metadata['png']['width']}}
 {%- endif %}
-{%- if 'height' in output.metadata.get('png', {}) %}
+{%- if 'metadata' in output and 'height' in output.metadata.get('png', {}) %}
 height={{output.metadata['png']['height']}}
 {%- endif %}
 >
@@ -131,10 +131,10 b" height={{output.metadata['png']['height']}}"
 
 {% block data_jpg %}
 <img src="data:image/jpeg;base64,{{ output.jpeg }}"
-{%- if 'width' in output.metadata.get('jpeg', {}) %}
+{%- if 'metadata' in output and 'width' in output.metadata.get('jpeg', {}) %}
 width={{output.metadata['jpeg']['width']}}
 {%- endif %}
-{%- if 'height' in output.metadata.get('jpeg', {}) %}
+{%- if 'metadata' in output and 'height' in output.metadata.get('jpeg', {}) %}
 height={{output.metadata['jpeg']['height']}}
 {%- endif %}
 >