variables: BIDSVariableCollection.to_df loses n/as
This relates to: https://github.com/poldracklab/fitlins/issues/56

If there are `n/a`s in a `Sparse` variable, and `get_design_collection` is called with `force=True`, missing values are ommited (even if `Analysis.setup` was called with `drop_na=False`, and the variables originally had the `n/a`s)

`BIDSVariableCollection.to_df` does have a `fillna` argument (which by default is `nan`), but the following line removes `n/a`s prior to this: 

https://github.com/INCF/pybids/blob/87015d897ab881d0db47994595d63bba06daa6ab/bids/variables/kollekshuns.py#L116

diff --git a/bids/variables/kollekshuns.py b/bids/variables/kollekshuns.py
index fdaf8f92..ab76c83d 100644
--- a/bids/variables/kollekshuns.py
+++ b/bids/variables/kollekshuns.py
@@ -113,9 +113,11 @@ class BIDSVariableCollection(object):
             return df.reset_index(drop=True).fillna(fillna)
 
         ind_cols = list(set(df.columns) - {'condition', 'amplitude'})
+
+        df['amplitude'] = df['amplitude'].fillna('n/a')
         df = df.pivot_table(index=ind_cols, columns='condition',
                             values='amplitude', aggfunc='first')
-        df = df.reset_index().fillna(fillna)
+        df = df.reset_index().replace('n/a', fillna)
         df.columns.name = None
         return df
 
['bids/variables/kollekshuns.py']