Monday fixes.
Address issue https://github.com/gnudist/littleorm/issues/7
More precisely address issue 7.
NULL SQL clause if undef
Revert prev change.
Correct handling of additional conditions passed in get_many() for joined filter:

my $f1 = M1 -> f();
my $f2 = M2 -> f();

$f1 -> connect_filter_left_outer_join( $f2, _clause => ... );

$f1 -> get_many( id => 123 );
Fix & test.
https://github.com/gnudist/littleorm/issues/5
Big cleanup in determine_op_and_col_and_correct_val() sub.
Fix & tests.