depthcharge: Fix non-fit boot without ramdisk

For non-fit kernels the ramdisk is passed on the command line; If no
ramdisk is used (which is valid) set the ramdisk value to "" otherwise
the line will be dropped from the commands

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>

lava_dispatcher/actions/boot/depthcharge.py

@@ -148,7 +148,7 @@ class DepthchargeCommandOverlay(BootloaderCommandOverlay):
         substitutions = {
             "{CMDLINE}": cmdline_tftp,
             "{DEPTHCHARGE_KERNEL}": fit_tftp or kernel_tftp,
-            "{DEPTHCHARGE_RAMDISK}": ramdisk_tftp if not fit_tftp else "",
+            "{DEPTHCHARGE_RAMDISK}": ramdisk_tftp or "" if not fit_tftp else "",
         }
         commands = self.get_namespace_data(
             action="bootloader-overlay", label=self.method, key="commands"