sql: exclude anonymous database from pg_catalog and pg_extension sche…

…mas in TableName.FQString()

Fixes #105929

`"".pg_extension.spatial_ref_sys` is the output of `TableName.FQString()` for
`spatial_ref_sys` which results in an error when trying to use the value:

```
[email protected]:26257/movr> SELECT count(*) FROM pg_extension.spatial_ref_sys;
  count
---------
   6139
(1 row)

Time: 17ms total (execution 17ms / network 0ms)

[email protected]:26257/movr> SELECT count(*) FROM "".pg_extension.spatial_ref_sys;
ERROR: cannot access virtual schema in anonymous database
SQLSTATE: 42704
HINT: verify that the current database is set
```

This PR fixes this by removing the `"".` prefix.

Release note: None

pkg/sql/sem/tree/table_name.go

@@ -10,6 +10,8 @@
 
 package tree
 
+import "github.com/cockroachdb/cockroach/pkg/sql/sem/catconstants"
+
 // TableName corresponds to the name of a table in a FROM clause,
 // INSERT or UPDATE statement, etc.
 //
@@ -46,8 +48,13 @@ func (t *TableName) objectName() {}
 // schema and catalog names. Suitable for logging, etc.
 func (t *TableName) FQString() string {
 	ctx := NewFmtCtx(FmtSimple)
-	ctx.FormatNode(&t.CatalogName)
-	ctx.WriteByte('.')
+	schemaName := t.SchemaName.String()
+	// The pg_catalog and pg_extension schemas cannot be referenced from inside
+	// an anonymous ("") database. This makes their FQ string always relative.
+	if schemaName != catconstants.PgCatalogName && schemaName != catconstants.PgExtensionSchemaName {
+		ctx.FormatNode(&t.CatalogName)
+		ctx.WriteByte('.')
+	}
 	ctx.FormatNode(&t.SchemaName)
 	ctx.WriteByte('.')
 	ctx.FormatNode(&t.ObjectName)