Post.text with strip_html=True fails for empty posts
```pytb
Scanning posts......done!
.  local_search:output/assets/js/tipuesearch_content.json
########################################
TaskError - taskid:local_search:output/assets/js/tipuesearch_content.json
PythonAction Error
Traceback (most recent call last):
  File "/home/kwpolska/virtualenvs/nikola-py3/lib/python3.5/site-packages/doit/action.py", line 383, in execute
    returned_value = self.py_callable(*self.args, **kwargs)
  File "/home/kwpolska/nikola-logs/plugins/localsearch/localsearch/__init__.py", line 77, in save_data
    text = post.text(lang, strip_html=True)
  File "/home/kwpolska/git/nikola/nikola/post.py", line 677, in text
    data = content.text_content().strip()  # No whitespace wanted.
  File "/home/kwpolska/virtualenvs/nikola-py3/lib/python3.5/site-packages/lxml/html/__init__.py", line 411, in text_content
    return _collect_string_content(self)
  File "src/lxml/xpath.pxi", line 433, in lxml.etree.XPath.__call__ (src/lxml/lxml.etree.c:168402)
  File "src/lxml/apihelpers.pxi", line 67, in lxml.etree._rootNodeOrRaise (src/lxml/lxml.etree.c:15759)
ValueError: Input object is not an XML element: HtmlComment
```

Caused by this file:

```html
<!--
.. title: Search
.. slug: search
.. date: 2015-07-20 14:18:28 UTC
.. tags:
.. category:
.. link:
.. description:
.. type: text
-->
```

To fix this, all I had to do was to add a newline (which was probably triggering the metadata comment removal thing), but we should protect against those crashes.
diff --git a/nikola/post.py b/nikola/post.py
index e564e52ea..701f2a179 100644
--- a/nikola/post.py
+++ b/nikola/post.py
@@ -687,7 +687,7 @@ class Post(object):
                 # Not all posts have a body. For example, you may have a page statically defined in the template that does not take content as input.
                 content = lxml.html.fromstring(data)
                 data = content.text_content().strip()  # No whitespace wanted.
-            except lxml.etree.ParserError:
+            except (lxml.etree.ParserError, ValueError):
                 data = ""
         elif data:
             if self.demote_headers:
['nikola/post.py']