Merged index is different if left-hand dataframe is empty
#### Code Sample

```python
import pandas as pd

pd.__version__
# u'0.23.4'

a = pd.DataFrame(columns=['refA', 'dataA'], data=[[1, 'bla']])
b = pd.DataFrame(columns=['refB', 'dataB'], data=[[2, 'pff']]).set_index('refB')
m = a.merge(b, left_on='refA', right_index=True)

# merged dataframe contains only column from dataframe `a`
m.index
# Index([], dtype='object')
m.columns
# Index([u'refA', u'dataA', u'dataB'], dtype='object')
m
# Empty DataFrame
# Columns: [refA, dataA, dataB]
# Index: []


a = pd.DataFrame(columns=['refA', 'dataA'])
b = pd.DataFrame(columns=['refB', 'dataB'], data=[[1, 'pff']]).set_index('refB')
m = a.merge(b, left_on='refA', right_index=True)

# merged dataframe contains column from dataframe `a` and index from dataframe `b`
m.index
# Index([], dtype='object', name=u'refB')
m.columns
# Index([u'refA', u'dataA', u'dataB'], dtype='object')
m
# Empty DataFrame
# Columns: [refA, dataA, dataB]
# Index: []
```
#### Problem description

When merging an empty dataframe with another dataframe on column and index respectively, the output contains both the index and the column. If the left dataframe is non-empty, then only the column from the left dataframe is carried through while the index from the right dataframe disappears.

#### Expected Output

Consistency between the case where the left dataframe is empty and non-empty, e.g. don't include the index from the righthand dataframe in the merged dataframe. This avoids having to explicitly handle cases where one dataframe is empty in the calling code.


#### Output of ``pd.show_versions()``
<details>
INSTALLED VERSIONS
------------------

commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.4
pytest: 3.0.5
pip: 18.0
setuptools: 39.2.0
Cython: 0.25.2
numpy: 1.14.2
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 5.1.0
sphinx: 1.7.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: None
sqlalchemy: 1.1.5
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.9.4
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
