BUG: Adding empty dataframes should result in empty blocks #10181
Fixes #10181 

I added a couple of lines to the dataframe test `test_operators` to assert that  adding two empty dataframes returns a dataframe that is equal (both using `assert_frame_equal` as well as `.equals`) to an empty dataframe.

I couldn't get vbench to install (I am running on windows) - the error was:

```
c:\dev\code\opensource\pandas-rekcahpassyla>pip install git+https://github.com/pydata/vbench
You are using pip version 6.0.8, however version 6.1.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Collecting git+https://github.com/pydata/vbench
  Cloning https://github.com/pydata/vbench to c:\users\redacted\appdata\local\temp\pip-ruiahg-build
fatal: unable to connect to github.com:
github.com[0: 192.30.252.129]: errno=No error

Clone of 'git://github.com/yarikoptic/vbenchtest.git' into submodule path 'vbench/tests/vbenchtest/vbenchtest' failed
  Complete output from command C:\dev\bin\git\cmd\git.exe submodule update --init --recursive -q:

  ----------------------------------------
  Command "C:\dev\bin\git\cmd\git.exe submodule update --init --recursive -q" failed with error code 1 in c:\users\redacted\appdata\local\temp\pip-ruiahg-build
```

The test suite failed for me in master with 1 failure. The same failure, but no others, were observed when running in my branch. 

Added more tests and comment as suggested
 @rekcahpassyla ok this looks reasonable. pls add a release note to whatsnew/v0.17.0 (you can put in other API changes section).

pls squash as well.
 Added release note and squashed.
 I note the build failure, will have a look. 
 I have verified that my latest commit passes the `pandas.stats` tests (the cause of the build failure). However I haven't squashed it, as I don't know if it would be considered a separate bug (searching the issues list didn't reveal anything). If it is a separate bug, I'm happy to open another issue and cherrypick this commit into another PR. 
 Raised  https://github.com/pydata/pandas/issues/10195 and proposed https://github.com/pydata/pandas/pull/10196 to fix the issue causing https://travis-ci.org/pydata/pandas/builds/63544612 to fail. 
 pls rebase this (and move the release note to 0.16.2)
 I think these are some old travis failures. can you rebase on master. ping when green.
 @jreback - It is green
 thanks!
