Cleaning output from string_agg

I am consolidating information from a view using

select
string_agg(mytable.notes::text, ','::text) AS notes,
.
.
.
from mytable, ...
where ...
groupby ...

Unfortunately some of the fields are blank rather than null giving the result "blah,,,,blah,"

How do I check the return for empty (it appears to work well for null) before the aggregation?

Thanks in advance