Generate valid GnuCash SQL by not inserting values if there are none.

src/pricehist/outputs/gnucashsql.py

@@ -66,6 +66,7 @@ class GnuCashSQL(BaseOutput):
             )
             values_parts.append(v)
         values = ",\n".join(values_parts)
+        values_comment = "" if values_parts else "-- "
 
         if too_big:
             # https://code.gnucash.org/docs/MAINT/group__Numeric.html
@@ -83,6 +84,7 @@ class GnuCashSQL(BaseOutput):
             timestamp=datetime.utcnow().isoformat() + "Z",
             base=self._sql_str(base),
             quote=self._sql_str(quote),
+            values_comment=values_comment,
             values=values,
         )
 

src/pricehist/resources/gnucash.sql

@@ -16,9 +16,9 @@ FROM prices p, commodities c
 WHERE FALSE;
 
 -- Populate the staging table.
-INSERT INTO new_prices (guid, date, base, quote, source, type, value_num, value_denom) VALUES
-{values}
-;
+{values_comment}INSERT INTO new_prices (guid, date, base, quote, source, type, value_num, value_denom) VALUES
+{values_comment}{values}
+{values_comment};
 
 -- Get some numbers for the summary.
 CREATE TEMPORARY TABLE summary (description TEXT, num INT);