ENH: Validate merge is not made on NA values in the merge keys
#### Is your feature request related to a problem?

I wish pandas merge function would prodive a way to verify that it does not merge on NA values, which, most of the times, are not what we want. Right now, I'm always doing a pre-check that my right dataframe does not have any NA values in keys, but when you are doing 10 merges one after the other, this is cumbersome and bug-prone. 

#### Describe the solution you'd like

This would be an additional parameter (e.g. validate_no_na_merge) in pd.merge() function, defaulting to False, which, when True, would just assert that there are no NA values in the keys of the right dataframe. Thus, checking that there won't be any merge on NA keys.

Note this leaves the possibility that keys in the _left_ dataframe contain NA values and will be kept as such in the final merged dataframe. Here we just wish to verify that the _merge_ itself is not made on NA values. 

#### API breaking implications

If the parameter defaults to False, I do not see any breaks.

#### Describe alternatives you've considered

Right now, I'm always doing a pre-check such as : 
`assert not df_right.merge_key.isnull().any()` 
`df = pd.merge(df_left, df_right, on='merge_key', ...)`

#### Additional context
None
