llvm-objdump: Don't print "Import table:" header if there's no import table.

If a binary does not depend on any DLL, it does not contain import table at
all. Printing the section title without contents looks wrong, so we shouldn't
print it in that case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199340 91177308-0d34-0410-b5e6-96231b3b80d8