[Fix #1243] Fix multiline-block cop which errors out on empty blocks

Currently, the multiline block cop errors out when it encounters code like this
```
x do
end
```
This commit checks for such blocks, and handles the condition correctly.
Merge pull request #1244 from vrthra/fix/1243/multiline-block-empty

[Fix #1243] Fix multiline-block cop which errors out on empty blocks