fix(spanner): use nil mutation for BeginTransaction

The BeginTransaction function requires a mutation to be passed in. This
should be nil when using regular sessions.

spanner/client.go

diff --git a/spanner/client.go b/spanner/client.go
index 3ad333c..b453194 100644
--- a/spanner/client.go
+++ b/spanner/client.go
@@ -1035,7 +1035,9 @@
 	t.txReadOnly.ro = c.ro
 	t.txReadOnly.disableRouteToLeader = c.disableRouteToLeader
 	t.txOpts = c.txo.merge(options)
-	if err := t.begin(ctx); err != nil {
+	// A mutation should only be included in the BeginTransaction RPC
+	// when using multiplexed sessions.
+	if err := t.begin(ctx /*mutation=*/, nil); err != nil {
 		sh.recycle()
 		return nil, err
 	}

spanner/client_test.go

diff --git a/spanner/client_test.go b/spanner/client_test.go
index 946d8c5..6393625 100644
--- a/spanner/client_test.go
+++ b/spanner/client_test.go
@@ -1954,15 +1954,17 @@
 			t.Fatal("missing transaction id")
 		}
 	}
-	// Verify that the next transaction fails, as the pool is exhausted and we have disabled the prepareFunc.
+	// Verify that the next transaction fails, as the pool is exhausted, and we have disabled the prepareFunc.
+	ctx, cancel := context.WithTimeout(ctx, 5*time.Millisecond)
+	defer cancel()
 	_, err = txPool.RunTransaction(ctx, func(ctx context.Context, transaction *ReadWriteTransaction) error {
 		return nil
 	})
 	if err == nil {
 		t.Fatal("missing error for last transaction")
 	}
-	if g, w := ErrCode(err), codes.ResourceExhausted; g != w {
-		t.Fatalf("error code mismatch\n Got %v\nWant: %v", g, w)
+	if g, w := err, context.DeadlineExceeded; !errors.Is(g, w) {
+		t.Fatalf("error mismatch\n Got: %v\nWant: %v", g, w)
 	}
 }
 
@@ -2039,15 +2041,17 @@
 			t.Fatal("missing transaction id")
 		}
 	}
-	// Verify that the next transaction fails, as the pool is exhausted and we have disabled the prepareFunc.
+	// Verify that the next transaction fails, as the pool is exhausted, and we have disabled the prepareFunc.
+	ctx, cancel := context.WithTimeout(ctx, 5*time.Millisecond)
+	defer cancel()
 	_, err = client.ReadWriteTransaction(ctx, func(ctx context.Context, transaction *ReadWriteTransaction) error {
 		return nil
 	})
 	if err == nil {
 		t.Fatal("missing error for last transaction")
 	}
-	if g, w := ErrCode(err), codes.ResourceExhausted; g != w {
-		t.Fatalf("error code mismatch\n Got %v\nWant: %v", g, w)
+	if g, w := err, context.DeadlineExceeded; !errors.Is(g, w) {
+		t.Fatalf("error mismatch\n Got: %v\nWant: %v", g, w)
 	}
 }