[838][scaledNcVar] Added isNull() to copy constructor

Squashed commit of the following:

commit 59deb773
Author: Jakob Lindo <jakob.c.lindo@nasa.gov>
Date:   Wed Apr 9 07:58:48 2025 -0400

    [838][libnetcdfutils] Updated test script to catch invalid_argument

commit 421882a7
Merge: c140aa20 0a08a686
Author: Jakob Lindo <jakob.c.lindo@nasa.gov>
Date:   Wed Apr 9 07:17:08 2025 -0400

    Merge branch 'develop' into 838-null-scaledncvar

commit c140aa20
Author: Jakob Lindo <jakob.c.lindo@nasa.gov>
Date:   Mon Apr 7 16:26:22 2025 -0400

    [838][scaledNcVar] Added isNull() to copy constructor