Fix "rally info" failure in case of empty docstrings

In case the input query for "rally info find ... " requested for a benchmark
scenario with an empty docstring, the CLI failed with an exception. Instead,
we should print that there is no info about that scenario.

Change-Id: If3a806f794f9b11ce1eb44eaabce732183c4c99b
Closes-Bug: 1371461
Merge "Fix "rally info" failure in case of empty docstrings"