Because it is not initialized, it is the default constructor that is called. Then :
empty string constructor (default constructor) :
Constructs an empty string, with a length of zero characters.
Take a look : http://www.cplusplus.com/reference/string/string/string/
EDIT :
As stated in C++11, §21.4.2/1 :
E?ects: Constructs an object of class basic_string. The postconditions of this function are indicated in Table 63.
-> Table 63
+-----------------------------------------------------------------------------+
| data() | a non-null pointer that is copyable and can have 0 added to it |
+------------+----------------------------------------------------------------+
| size() | 0 |
+------------+----------------------------------------------------------------+
| capacity() | an unspeci?ed value |
+-----------------------------------------------------------------------------+
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