fix(code): Ensure all studies are part of intervention df

Some studies were missing crucial fields (prelim studies mostly) which means they
are dropped from being counted in 'by_intervention' aggregation. While
ultimately those will have to have their fields filled correctly, for now we can
simply ensure all studies are aggregated correctly per intervention.

scoping_review.qmd

@@ -565,7 +565,9 @@ analysing the main findings per policy area, as well as underscore individual st
 #| fig-cap: Available studies by primary type of intervention
 
 by_intervention = (
-    bib_df.groupby(["author", "year", "title", "design", "method", "representativeness", "citation"])
+    bib_df
+    .fillna("")
+    .groupby(["author", "year", "title", "design", "method", "representativeness", "citation"])
     .agg(
         {
             "intervention": lambda _col: "; ".join(_col),