solve bug with no distortion

[BenjaminKrummenacher]

Solves bug that occurred when running 'optvi' in the maplab_console when using a calibration file with no distortion:

The nullptr which was added to cost_term_args when the distortion is of type kNoDistortion would later fail the following check, since the sizes don't match: [problem_impl.cc:293] Check failed: parameter_blocks.size() == cost_function->parameter_block_sizes().size() (9 vs. 8)

My solution avoids adding that nullptr if there is no distortion.

[ethzasl-jenkins]

Can one of the admins verify this patch?

[ethzasl-jenkins]

Test PASSed.

[ethzasl-jenkins]

Test PASSed.

[ethzasl-jenkins]

Test PASSed.

[ethzasl-jenkins]

Test FAILed.

[ethzasl-jenkins]

Test PASSed.