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. 
