fix: infer amountless unbalanced virtual postings as zero

They were effectively zero already, but print always showed them with no amount, even with -x, which was unclear.

hledger-lib/Hledger/Data/Balancing.hs

@@ -241,6 +241,7 @@ transactionInferBalancingAmount styles t@Transaction{tpostings=ps}
         minferredamt = case ptype p of
           RegularPosting         | not (hasAmount p) -> Just realsum
           BalancedVirtualPosting | not (hasAmount p) -> Just bvsum
+          VirtualPosting         | not (hasAmount p) -> Just 0
           _                                          -> Nothing
       in
         case minferredamt of

hledger/test/journal/virtual-postings.test

@@ -32,18 +32,16 @@ $ hledger -f- print
 2010/1/1 x
   [a]    10
   [b]
-$ hledger -f- print
+$ hledger -f- print -x
 > //
 
-# 5. real and balanced virtual postings are balanced separately, and multiple blank virtuals are ok
+# 5. real and balanced virtual postings are balanced separately, and multiple amountless virtuals are ok
 <
 2010/1/1 x
   a  1
   b
   [e]  10
   [f]
-  (c)
-  (d)
 $ hledger -f- balance --tree
                    1  a
                   -1  b
@@ -51,3 +49,17 @@ $ hledger -f- balance --tree
                  -10  f
 --------------------
                    0  
+
+# 6. Amountless unbalanced postings are always inferred as zero.
+<
+2023-01-01
+  (a)
+  (b)
+  (c)
+$ hledger -f- print -x
+2023-01-01
+    (a)               0
+    (b)               0
+    (c)               0
+
+>=