SSE: Fix math expression to support NoData results (#61721)

* update perFloat to support NoData
* update union to correctly handle no-data

pkg/expr/mathexp/union_test.go

@@ -103,7 +103,13 @@ func Test_union(t *testing.T) {
 				},
 			},
 			unionsAre: assert.EqualValues,
-			unions:    []*Union{},
+			unions: []*Union{
+				{
+					Labels: nil,
+					A:      makeSeries("a", data.Labels{"id": "1"}),
+					B:      NewNoData(),
+				},
+			},
 		},
 		{
 			name: "incompatible tags of different length with will result in no unions when len(A) != 1 && len(B) != 1",
@@ -260,6 +266,30 @@ func Test_union(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "A is no-data and B is anything makes no-data",
+			// Is this the behavior we want? A result within the results will no longer
+			// be uniquely identifiable.
+			aResults: Results{
+				Values: Values{
+					NewNoData(),
+				},
+			},
+			bResults: Results{
+				Values: Values{
+					makeSeries("a", data.Labels{"id": "1"}),
+					makeSeries("aa", data.Labels{"id": "1", "fish": "herring"}),
+				},
+			},
+			unionsAre: assert.EqualValues,
+			unions: []*Union{
+				{
+					Labels: nil,
+					A:      NewNoData(),
+					B:      makeSeries("a", data.Labels{"id": "1"}),
+				},
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {