gh-39681: Fix eulerian_number(0,0) and eulerian_polynomial(0) to return 1
    
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This PR fixes a bug in the `eulerian_number` and `eulerian_polynomial`
functions where `eulerian_number(0, 0)` and `eulerian_polynomial(0)`
incorrectly return `0` instead of `1`. The change ensures these
functions align with the standard combinatorial definitions of Eulerian
numbers and polynomials.

Fixes: #39679

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preview.

### ⌛ Dependencies

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URL: #39681
Reported by: Vidip Singh
Reviewer(s):

Author: Release Manager

src/sage/combinat/combinat.py

@@ -562,6 +562,8 @@ def eulerian_number(n, k, algorithm='recursive') -> Integer:
         [0, 1, 4, 1, 0]
     """
     n = ZZ(n)
+    if n == 0:
+        return ZZ.one() if k == 0 else ZZ.zero()
     if k < 0 or k > n - 1:
         return ZZ.zero()
     if k == 0 or k == n - 1:
@@ -614,7 +616,7 @@ def eulerian_polynomial(n, algorithm='derivative'):
     R = PolynomialRing(ZZ, 't')
     if n < 0:
         return R.zero()
-    if n == 1:
+    if n <= 1:
         return R.one()
     t = R.gen()
     if algorithm == 'derivative':