Fix uninitialized variable warning when not using LTI.

[somiaj]

Check that $ce->{LTIVersion} is defined and not empty before using it. This variable being initialized should ensure that the other LTI variables used here are also initialized.

[drgrice1]

Good catch. I don't know how I missed this when this change was added. Probably the fact that I typically have LTI authentication enabled.

[somiaj]

@drgrice1 Think we can remove the line && defined $ce->{LTIGradeMode}, since if $ce->{LTIVersion} is defined, this not being defined is probably an error?

[drgrice1]

In theory LTIGradeMode does not need to be defined even if the LTIVersion is. I think that all of the webwork2 code is designed to check that it is defined first. Note that there are many places where it is not directly checked that LTIGradeMode is defined and it is just used and compared to a string, but the code in those regions can only be called if it is defined.