kvdb/test: fix cursor tests to match bbolt semantics

From bbolt docs:
// Seek positions the cursor at the passed seek key. If the key does not exist,
// the cursor is moved to the next key after seek. Returns the new pair.

kvdb/etcd/readwrite_cursor.go

@@ -96,11 +96,6 @@ func (c *readWriteCursor) Prev() (key, value []byte) {
 // not exist, the cursor is moved to the next key after seek.  Returns
 // the new pair.
 func (c *readWriteCursor) Seek(seek []byte) (key, value []byte) {
-	// Return nil if trying to seek to an empty key.
-	if seek == nil {
-		return nil, nil
-	}
-
 	// Seek to the first key with prefix + seek. If that key is not present
 	// STM will seek to the next matching key with prefix.
 	kv, err := c.bucket.tx.stm.Seek(c.prefix, c.prefix+string(seek))

kvdb/readwrite_cursor_test.go

@@ -99,8 +99,8 @@ func testReadCursorNonEmptyInterval(t *testing.T, db walletdb.DB) {
 
 		// Seek to nonexisting key.
 		k, v = cursor.Seek(nil)
-		require.Nil(t, k)
-		require.Nil(t, v)
+		require.Equal(t, "b", string(k))
+		require.Equal(t, "1", string(v))
 
 		k, v = cursor.Seek([]byte("x"))
 		require.Nil(t, k)
@@ -113,7 +113,6 @@ func testReadCursorNonEmptyInterval(t *testing.T, db walletdb.DB) {
 }
 
 func testReadWriteCursor(t *testing.T, db walletdb.DB) {
-
 	testKeyValues := []KV{
 		{"b", "1"},
 		{"c", "2"},