fix fft axis (#36321)

fix: `-1` is used when fft's axis is `0`

python/paddle/tensor/fft.py

@@ -1340,7 +1340,7 @@ def fft_c2c(x, n, axis, norm, forward, name):
         x = paddle.cast(x, _real_to_complex_dtype(x.dtype))
     _check_normalization(norm)
 
-    axis = axis or -1
+    axis = axis if axis is not None else -1
     _check_fft_axis(x, axis)
     axes = [axis]
     axes = _normalize_axes(x, axes)
@@ -1370,7 +1370,7 @@ def fft_r2c(x, n, axis, norm, forward, onesided, name):
     if is_interger(x):
         x = paddle.cast(x, paddle.get_default_dtype())
     _check_normalization(norm)
-    axis = axis or -1
+    axis = axis if axis is not None else -1
     _check_fft_axis(x, axis)
     axes = [axis]
     axes = _normalize_axes(x, axes)
@@ -1409,7 +1409,7 @@ def fft_c2r(x, n, axis, norm, forward, name):
     elif is_floating_point(x):
         x = paddle.cast(x, _real_to_complex_dtype(x.dtype))
     _check_normalization(norm)
-    axis = axis or -1
+    axis = axis if axis is not None else -1
     _check_fft_axis(x, axis)
     axes = [axis]
     axes = _normalize_axes(x, axes)