[TIFF writing] Fix an assumption that was broken by a prior commit.

r14653 ensured that we never created a TaggedImage with null metadata, to fix
a downstream problem. However, the fixIndexMap() function explicitly checks for
null metadata to detect placeholder images. It has now been modified to also
check for zero-length metadata and also treat that as a placeholder; in
practice I do not think we will ever have real images with zero-length
metadata, so this should be safe, especially as fixIndexMap() is rarely
called.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14802 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd