gh-35610: Check for empty string before trying to evaluate in _elemen…

…t_constructor

    
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Fixes #35607
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URL: #35610
Reported by: nbruin
Reviewer(s): Matthias Köppe

src/sage/rings/complex_mpfr.pyx

@@ -517,7 +517,7 @@ class ComplexField_class(sage.rings.abc.ComplexField):
                 x = x.replace('j', 'I')
                 x = x.replace('E', 'e')
                 allowed = '+-.*0123456789Ie'
-                if not all(letter in allowed for letter in x):
+                if len(x) == 0 or not all(letter in allowed for letter in x):
                     raise ValueError(f'given string {x!r} is not a complex number')
                 # This should rather use a proper parser to validate input.
                 # TODO: this is probably not the best and most