Replace "" (empty string) default values of multiple options with "null".

This is preparation for turning on parsing of default values for multiple (allowMultiple = true) options.
This change is NOOP (enforced by OptionDefaultValueConversionTest#shouldConvertDefaultValue()) because for now default value of a multiple option does not matter and is always converted to an empty list. But it will matter a bit later: "null" - special value resolving to the same empty list, anything else - parsed by the converter.

RELNOTES: None.
PiperOrigin-RevId: 303345588
diff --git a/src/main/java/com/google/devtools/build/lib/exec/ExecutionOptions.java b/src/main/java/com/google/devtools/build/lib/exec/ExecutionOptions.java
index fdecd0b..1a1c1bc 100644
--- a/src/main/java/com/google/devtools/build/lib/exec/ExecutionOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/exec/ExecutionOptions.java
@@ -88,7 +88,7 @@
       name = "strategy",
       allowMultiple = true,
       converter = Converters.StringToStringListConverter.class,
-      defaultValue = "",
+      defaultValue = "null",
       documentationCategory = OptionDocumentationCategory.UNCATEGORIZED,
       effectTags = {OptionEffectTag.UNKNOWN},
       help =
@@ -105,7 +105,7 @@
       converter = RegexFilterAssignmentConverter.class,
       documentationCategory = OptionDocumentationCategory.UNCATEGORIZED,
       effectTags = {OptionEffectTag.UNKNOWN},
-      defaultValue = "",
+      defaultValue = "null",
       help =
           "Override which spawn strategy should be used to execute spawn actions that have "
               + "descriptions matching a certain regex_filter. See --per_file_copt for details on"