Application hist outputs bad stats when all DNs are special pixels

[scsides]

All versions

Description
The application "hist" outputs valid looking statistics when the input cube is full of special pixels.

How to reproduce

makecube pixels=NULL to=junk.cub samples=10 lines=5 bands=1
hist from=junk.cub to=junk.txt
cat junk.txt

Cube:           8.cub
Band:           1
Average:        -1.79769e+308
Std Deviation:  -1.79769e+308
Variance:       -1.79769e+308
Median:         -1.79769e+308
Mode:           -1.79769e+308
Skew:           -1.79769e+308
Minimum:        -1.79769e+308
Maximum:        -1.79769e+308

Total Pixels:    120
Valid Pixels:    0
Null Pixels:     120
Lis Pixels:      0
Lrs Pixels:      0
His Pixels:      0
Hrs Pixels:      0

Possible Solution
Test the returned value against ISIS::NULL when hist gets the stats. Print N/A instead of -1.79769e+308. It is an edge case, but we should not print a legal looking number when the return is trying to tell the programmer there is a problem.

Additional context

[jessemapel]

Not sure if this is a bug or enhancement, but I'm just going with enhancement as the program is correctly ignoring special pixels, it just returns the double value of ISIS:null instead of a more user friendly output when there are no valid pixels.

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[jessemapel]

This is a good thing to still do since we just did this for stats in #3895