Try to prevent spaces from being tagged as entities

spacy/syntax/ner.pyx

@@ -10,6 +10,8 @@ from ._state cimport StateC
 from .transition_system cimport Transition
 from .transition_system cimport do_func_t
 from ..gold cimport GoldParseC, GoldParse
+from ..lexeme cimport Lexeme
+from ..attrs cimport IS_SPACE
 from ..errors import Errors
 
 
@@ -273,6 +275,9 @@ cdef class Begin:
         # Don't allow entities to extend across sentence boundaries
         elif st.B_(1).sent_start == 1:
             return False
+        # Don't allow entities to start on whitespace
+        elif Lexeme.get_struct_attr(st.B_(0).lex, IS_SPACE):
+            return False
         else:
             return label != 0 and not st.entity_is_open()
 
@@ -366,6 +371,9 @@ cdef class Last:
     cdef bint is_valid(const StateC* st, attr_t label) nogil:
         if st.B_(1).ent_iob == 1:
             return False
+        # Don't allow entities to end on whitespace
+        elif Lexeme.get_struct_attr(st.B_(0).lex, IS_SPACE):
+            return False
         return st.entity_is_open() and label != 0 and st.E_(0).ent_type == label
 
     @staticmethod
@@ -418,6 +426,8 @@ cdef class Unit:
             return False
         elif st.B_(1).ent_iob == 1:
             return False
+        elif Lexeme.get_struct_attr(st.B_(0).lex, IS_SPACE):
+            return False
         return label != 0 and not st.entity_is_open()
 
     @staticmethod