imp: journal: allow an empty double-quoted commodity symbol

"" is equivalent to writing no symbol.
But it's now possible to declare market prices for the no-symbol
commodity, which occurs eg with timedot data.

P 2022-01-01 "" $100

hledger-lib/Hledger/Read/Common.hs

@@ -831,7 +831,7 @@ commoditysymbolp =
 
 quotedcommoditysymbolp :: TextParser m CommoditySymbol
 quotedcommoditysymbolp =
-  between (char '"') (char '"') $ takeWhile1P Nothing f
+  between (char '"') (char '"') $ takeWhileP Nothing f
   where f c = c /= ';' && c /= '\n' && c /= '\"'
 
 simplecommoditysymbolp :: TextParser m CommoditySymbol

hledger/test/ledger-compat/syntax/hledger-other.test

@@ -54,6 +54,7 @@ assert 1
 bucket Assets:Checking
 A Assets:Checking
 C $1 = USD 1
+C 1 "" = $1
 capture Expenses:Deductible:Medical Medical
 check 1
 define var_name=$100