fix(stdlib/influxdata/wideto): panic with null string key (#5493)

Update the wideTo function to cope with tables with a null tag
value. In these cases the tag is skipped when writing to influxdb.

stdlib/influxdata/influxdb/wide_to.go

@@ -231,6 +231,11 @@ func getTablePointsMetadata(tbl flux.Table) (md tablePointsMetadata, err error)
 			}
 			isTag[col.Label] = true
 
+			// If the tag is NULL then skip over adding its value to the tag set.
+			if tbl.Key().IsNull(j) {
+				continue
+			}
+
 			value := tbl.Key().ValueString(j)
 			if value == "" {
 				// Skip tag value if it is empty.

stdlib/influxdata/influxdb/wide_to_test.go

@@ -134,7 +134,7 @@ func TestWideToTransformation(t *testing.T) {
 		static.Ints("f1", 1, nil, 3, nil),
 		static.StringKey("_measurement", "m0"),
 		static.TableList{
-			static.StringKeys("t0", "a", "b"),
+			static.StringKeys("t0", "a", nil, "b"),
 		},
 	}
 	if err := input.Do(func(tbl flux.Table) error {
@@ -147,6 +147,9 @@ func TestWideToTransformation(t *testing.T) {
 	want := `m0,t0=a f0=1,f1=1i 0
 m0,t0=a f0=2 10000000000
 m0,t0=a f0=3,f1=3i 20000000000
+m0 f0=1,f1=1i 0
+m0 f0=2 10000000000
+m0 f0=3,f1=3i 20000000000
 m0,t0=b f0=1,f1=1i 0
 m0,t0=b f0=2 10000000000
 m0,t0=b f0=3,f1=3i 20000000000