What is the evidence that mathematicians are more likely to have autistic traits than the rest of the population?
No. I can't seem to find a specific study into professional mathematicians or graduate students so far, the studies always seem to group the occupations together. 

However, Baron-Cohen's team has done an impressive national survey (Ruzich et al., 2015) and recruited nearly half a million people through a UK TV show. They managed to get over 450,394 survey respondents which allowed them to analyse occupations. They found that people working in STEM scored higher in an autism screening questionnaire or AQ (Autism-Spectrum Quotient) test. 


  individuals working in a STEM career (m = 21.92, SD = 8.92) scored higher than individuals non-STEM careers (m = 18.92, SD = 8.48)


Unfortunately, there wasn't a specific category for mathematicians as probably it is too specific and too small a field to analyse. The three categories (figure 3) forming STEM was Comp &amp; IT, Engineering and Sci &amp; Tech. Mathematicians would fit into one of those occupational roles. 

References

Ruzich, E., Allison, C., Chakrabarti, B., Smith, P., Musto, H., Ring, H., &amp; Baron-Cohen, S. (2015). Sex and STEM Occupation Predict Autism-Spectrum Quotient (AQ) Scores in Half a Million People. PLOS ONE, 10(10), e0141229.