Fix an unexpected warning from makewindows

[hisakatha]

Fix an issue where makewindows with a genome option (-g) and a count option (-n)
will always raise a warning of a zero length interval, which is appended by
NewGenomeFile class

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I have found that bedtools makewindows with genome (-g) and count (-n) options always raise a warning of a zero length interval.
For example,

$ bedtools makewindows -g <(echo -e "chromA\t8") -n 3
chromA  0       2
chromA  2       4
chromA  4       8
WARNING: Interval :0-0 is smaller than the number of windows requested. Skipping.

The warning does not affect the standard output results, but it is not related to user-defined inputs and seems to be confusing.

The warning is written at

bedtools2/src/windowMaker/windowMaker.cpp

Line 107 in 484c0d4

if (window_size == 0)

Then, I noticed that a zero length chromosome was appended by NewGenomeFile class, but not ignored by WindowMaker class.
Although GenomeFile class does not append a zero length chromosome and seems to work, NewGenomeFile class provides the stdin feature (commit 43ac847 and issue #735), so I avoided reverting to GenomeFile class from NewGenomeFile class at

bedtools2/src/windowMaker/windowMaker.cpp

Line 47 in 484c0d4

NewGenomeFile *_genome = new NewGenomeFile(genomeFileName);

However, this fix relies on the order of the empty chromosome from the NewGenomeFile object, so it may be better to backport the stdin support to GenomeFile class for stability.
In addition, my English may be incorrect, so please feel free to edit my commit and comments.

Environment:
bedtools (v2.30.0 and the current master 484c0d4)
Ubuntu 20.04.2