Fix TODO in sympy/functions/special/spherical_harmonics.py

[anhnguyenphung]

Fix 'TODO: Assert |m| <= n otherwise we should return 0' in sympy/functions/special/spherical_harmonics.py

Add the case |m| > n to return 0 in _eval_rewrite_as_polynomial() and _eval_rewrite_as_cos() of the class Ynm(Function)

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#### Fix 'TODO: Assert |m| <= n otherwise we should return 0' in sympy/functions/special/spherical_harmonics.py

#### Add the case |m| > n to return 0 in _eval_rewrite_as_polynomial() and _eval_rewrite_as_cos() of the class Ynm(Function)

#### Release Note

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<!-- END RELEASE NOTES -->

[sidhu1012]

@anhnguyenphung It would be better to add test cases for your changes.

[anhnguyenphung]

@sidhu1012 I have just added the test cases in sympy/functions/special/tests/test_spherical_harmonics.py to test my change.

[JSS95]

Any more review?

[smichr]

We should return SymPy objects from routines like this.

[smichr]

If this should handle symbolic m,n then the test should be if (abs(m) - n).is_positive and a test for an indeterminate symbolic case should be added to the tests.