Check for empty string before trying to evaluate in _element_constructor

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

src/sage/rings/real_double.pyx

@@ -1522,7 +1522,7 @@ cdef class RealDoubleElement(FieldElement):
     def round(self):
         """
         Round ``self`` to the nearest integer.
-        
+
         This uses the convention of rounding half to even
         (i.e., if the fractional part of ``self`` is `0.5`, then it
         is rounded to the nearest even integer).

src/sage/rings/real_mpfr.pyx

@@ -2992,7 +2992,7 @@ cdef class RealNumber(sage.structure.element.RingElement):
         """
          Round ``self`` to the nearest representable integer, rounding halfway
          cases away from zero.
-         
+
          .. NOTE::
 
              The rounding mode of the parent field does not affect the result.