ovsdb-idl: Fix row insertion and deletion behavior.

ovsdb-idl: Fix row insertion and deletion behavior.

When the IDL was used to insert a row, but all of the new row's columns
were left at the default values, then the IDL would not insert the row at
all.

When the IDL was used to delete one or more rows, and the transaction did
not include any update or insertion operations, the transaction was dropped
entirely.

This commit fixes these two bugs.  It needs a regression test, but this
commit does not add one.