collect(): Do not ignore null values

Closes #5039

Author: mattnibs

docs/tutorials/schools.md

@@ -1013,7 +1013,8 @@ and produces
     Schools: [
         "Auberry Elementary",
         "Balch Camp Elementary",
-        "Pine Ridge Elementary"
+        "Pine Ridge Elementary",
+        null (string)
     ]
 }
 {
@@ -1023,9 +1024,7 @@ and produces
         "www.bigcreekschool.com"
     ],
     Schools: [
-        "Big Creek Elementary"
-    ]
-}
+        "Big Creek Elementary",
 ...
 ```
 

runtime/sam/expr/agg/collect.go

@@ -16,9 +16,7 @@ type Collect struct {
 var _ Function = (*Collect)(nil)
 
 func (c *Collect) Consume(val zed.Value) {
-	if !val.IsNull() {
-		c.update(val)
-	}
+	c.update(val)
 }
 
 func (c *Collect) update(val zed.Value) {

runtime/sam/expr/agg/ztests/collect-untag-unions.yaml

@@ -4,6 +4,7 @@ input: |
   1((int64,string))
   "foo"((time,string))
   0.5
+  null(string)
 
 output: |
-  [1,"foo",0.5]
+  [1,"foo",0.5,null]