Fix for bug #21309, the 'R2eff' dispersion model failure when peak intensity data is missing.
The bug is reported at https://gna.org/bugs/?21309.
The problem was that the check for missing data in the _calculate_r2eff() private API method was
accidentally deleted in r21505 in the relax_disp branch. See the commit at
http://article.gmane.org/gmane.science.nmr.relax.scm/19261 and the accidental deletion at
http://svn.gna.org/viewcvs/relax/branches/relax_disp/specific_analyses/relax_disp/api.py?view=diff&r1=21504&r2=21505&pathrev=21505.
