handle nil line to return something useful (bnc#886843)

src/include/bootloader/routines/misc.rb

@@ -473,6 +473,7 @@ def getKernelParamFromLine(line, key)
     #   "true" to add key without value
     # @return [String] new kernel command line
     def setKernelParamToLine(line, key, value)
+      line ||= ""
       # FIXME this doesn't work with quotes and spaces
       params = Builtins.splitstring(line, " ")
       params = Builtins.filter(params) { |p| p != "" }

test/boot_common_test.rb

@@ -45,5 +45,26 @@
       expect(Yast::BootCommon.setKernelParamToLine(
         line, "verbose", "false")).to eq result_line
     end
+
+    it "return line with key=value if line is nil" do
+      line = nil
+      result_line = "silent=1"
+      expect(Yast::BootCommon.setKernelParamToLine(
+        line, "silent", "1")).to eq result_line
+    end
+
+    it "return line with key when value is \"true\" and line is nil" do
+      line = nil
+      result_line = "verbose"
+      expect(Yast::BootCommon.setKernelParamToLine(
+        line, "verbose", "true")).to eq result_line
+    end
+
+    it "return empty string when value is \"false\" and line is nil" do
+      line = nil
+      result_line = ""
+      expect(Yast::BootCommon.setKernelParamToLine(
+        line, "verbose", "false")).to eq result_line
+    end
   end
 end