Find tables with columns with empty and NULL values in Postgresql
Assuming &quot;empty&quot; and &quot;blank values&quot; means empty string ('').
This function checks whether the passed table has any NULL or empty values ('') in the passed column (which must be a string type or some other type where the empty string is valid; not the case for numeric types for instance):
CREATE FUNCTION f_has_missing(_tbl regclass, _col text, OUT has_missing boolean)
  LANGUAGE plpgsql AS
$func$
BEGIN
   EXECUTE
   format($$SELECT EXISTS (SELECT FROM %s WHERE %2$I = '' OR %2$I IS NULL)$$, _tbl, _col)
   INTO has_missing;
END
$func$;

Call:
SELECT f_has_missing('tablename', 'column')

Or, optionally, schema-qualified:
SELECT f_has_missing('schema.tablename', 'column')

db&lt;&gt;fiddle here
Probably most important: Never concatenate parameter values into SQL code blindly. That's begging for SQL injection. I sanitized the code with format(). See:

Table name as a PostgreSQL function parameter

There are a couple of smart expressions to check for both:
(string_col = '') IS NOT FALSE
(string_col &lt;&gt; '') IS NOT TRUE
coalesce(string_col, '') = ''

See:

Best way to check for “empty or null value”

But I chose this plain and more verbose expression for two reason:
string_col = '' OR string_col IS NULL

I have grown fond of simple, obvious code, and none of the above is as clear as this. But more importantly, this expression can use an index on (string_col), while the above cannot - which makes a big difference for big tables. See the added demo in the fiddle!
db&lt;&gt;fiddle here
Obviously, we want a different name for the function than &quot;has_nulls&quot;, now.
I use an OUT parameter for convenience and short code. Now we can assign to it and be done.
And I use regclass as IN parameter for the table name. This way I can provide a schema explicitly or not. Again, see:

Table name as a PostgreSQL function parameter

Aside, one might loop through all columns of a table or a whole db to find any such column at once. Related:

Replace empty strings with null values
Set empty strings ('') to NULL in the whole database