opt: fix plan gist decoding of inverted filters

Details about inverted filter nodes are not encoded in plan gists. The
explain emitter assumed there were always some details encoded, and
would raise an internal error whenever decoding a plan gist with an
inverted filter. This commit prevents the internal error from occurring.

Fixes #108979

There is no release not because plan gists are an undocumented feature.

Release note: None

pkg/sql/opt/exec/execbuilder/testdata/explain_gist

@@ -171,3 +171,21 @@ SELECT crdb_internal.decode_plan_gist('AgGSARIAAwlAsJ8BE5IBAhcGFg==')
           table: ?@?
           spans: 1+ spans
           limit
+
+# Regression test for #108979. Correctly decode inverted filters.
+query T nosort
+SELECT crdb_internal.decode_plan_gist('AgGwAgQAgQIAAgAEBQITsAICAxgGDA==')
+----
+• top-k
+│ order
+│
+└── • filter
+    │
+    └── • index join
+        │ table: ?@?
+        │
+        └── • inverted filter
+            │
+            └── • scan
+                  table: ?@?
+                  spans: 1+ spans

pkg/sql/opt/exec/explain/emit.go

@@ -780,8 +780,12 @@ func (e *emitter) emitNodeAttributes(n *Node) error {
 
 	case invertedFilterOp:
 		a := n.args.(*invertedFilterArgs)
-		ob.Attr("inverted column", a.Input.Columns()[a.InvColumn].Name)
-		ob.Attr("num spans", len(a.InvFilter.SpansToRead))
+		if a.InvColumn != 0 {
+			ob.Attr("inverted column", a.Input.Columns()[a.InvColumn].Name)
+		}
+		if a.InvFilter != nil && len(a.InvFilter.SpansToRead) > 0 {
+			ob.Attr("num spans", len(a.InvFilter.SpansToRead))
+		}
 
 	case invertedJoinOp:
 		a := n.args.(*invertedJoinArgs)