Task: Fix the calculation for identity operator's expectation value (backport #13345) (Qiskit/qiskit#13436)

org: Qiskit
repo: qiskit
pr_number: 13436
issue_title: Fix the calculation for identity operator's expectation value (backport #13345)
issue_body: \n\n### Summary\n\nFix a bug in `qiskit.quantum_info` in the calculation of expectation value of the identity operator.\n\nFixes #13029 \n\n\n\n### Details and comments\nCurrently,  `qiskit.quantum_info` yields incorrect results for the expectation value of an identity operator for a given state vector. It returns the norm of the state vector; however, it ought to return the square of the norm.\n\nTherefore, I am changing the following line  \n\nhttps://github.com/Qiskit/qiskit/blob/1be2c5aa103f1991f2fdc75a77a09f82635b8431/qiskit/quantum_info/states/statevector.py#L478-L479\n\nto \n```Python\n   if x_mask + z_mask == 0: \n       return pauli_phase * np.linalg.norm(self.data) ** 2\n```\n<hr>This is an automatic backport of pull request #13345 done by [Mergify](https://mergify.com).
resolved_issues: number: 13029
title: Statevector._expectation_value_pauli returns incorrect result in certain cases
body: The calculation for the expectation value of the __identity operator__ seems incorrect when the statevector is __not normalized__ to 1.\r\n\r\nI think the reason is that one should use the norm squared rather than the norm in the following lines.\r\nhttps://github.com/Qiskit/qiskit/blob/1be2c5aa103f1991f2fdc75a77a09f82635b8431/qiskit/quantum_info/states/statevector.py#L478-L479\r\n\r\n---\r\n\r\nThe whole function is attached below to provide context.\r\n\r\nhttps://github.com/Qiskit/qiskit/blob/1be2c5aa103f1991f2fdc75a77a09f82635b8431/qiskit/quantum_info/states/statevector.py#L458-L491

Reference solution:
fix_patch: diff --git a/qiskit/quantum_info/states/statevector.py b/qiskit/quantum_info/states/statevector.py\nindex 901ce95af424..1265d677abe4 100644\n--- a/qiskit/quantum_info/states/statevector.py\n+++ b/qiskit/quantum_info/states/statevector.py\n@@ -476,7 +476,7 @@ def _expectation_value_pauli(self, pauli, qargs=None):\n         pauli_phase = (-1j) ** pauli.phase if pauli.phase else 1\n \n         if x_mask + z_mask == 0:\n-            return pauli_phase * np.linalg.norm(self.data)\n+            return pauli_phase * np.linalg.norm(self.data) ** 2\n \n         if x_mask == 0:\n             return pauli_phase * expval_pauli_no_x(self.data, self.num_qubits, z_mask)\n