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

org: Qiskit
repo: qiskit
pr_number: 13345
issue_title: Fix the calculation for identity operator's expectation value
issue_body: <!--\r\n⚠️ If you do not respect this template, your pull request will be closed.\r\n⚠️ Your pull request title should be short detailed and understandable for all.\r\n⚠️ Also, please add a release note file using reno if the change needs to be\r\n  documented in the release notes.\r\n⚠️ If your pull request fixes an open issue, please link to the issue.\r\n\r\n- [ ] I have added the tests to cover my changes.\r\n- [ ] I have updated the documentation accordingly.\r\n- [ x] I have read the CONTRIBUTING document.\r\n-->\r\n\r\n### Summary\r\n\r\nFix a bug in `qiskit.quantum_info` in the calculation of expectation value of the identity operator.\r\n\r\nFixes #13029 \r\n\r\n\r\n\r\n### Details and comments\r\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.\r\n\r\nTherefore, I am changing the following line  \r\n\r\nhttps://github.com/Qiskit/qiskit/blob/1be2c5aa103f1991f2fdc75a77a09f82635b8431/qiskit/quantum_info/states/statevector.py#L478-L479\r\n\r\nto \r\n```Python\r\n   if x_mask + z_mask == 0: \r\n       return pauli_phase * np.linalg.norm(self.data) ** 2\r\n```\r\n
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