Schema - don't make default null for null values.

Closes nf-core#1074

CHANGELOG.md

@@ -7,6 +7,10 @@
 * Fixed a bug in the Docker image build for tools that failed due to an extra hyphen. [[#1069](https://github.com/nf-core/tools/pull/1069)]
 * Regular release sync fix - this time it was to do with JSON serialisation [[#1072](https://github.com/nf-core/tools/pull/1072)]
 
+#### Sync
+
+* Don't set the default value to `"null"` when a parameter is initialised as `null` in the config [[#1074](https://github.com/nf-core/tools/pull/1074)]
+
 ## [v1.14 - Brass Chicken :chicken:](https://github.com/nf-core/tools/releases/tag/1.14) - [2021-05-11]
 
 ### Template

nf_core/schema.py

@@ -526,6 +526,10 @@ def build_schema_param(self, p_val):
         except ValueError:
             p_type = "string"
 
+        # Anything can be "null", means that it is not set
+        if p_val == "null":
+            p_val = None
+
         # NB: Only test "True" for booleans, as it is very common to initialise
         # an empty param as false when really we expect a string at a later date..
         if p_val == "True":
@@ -535,7 +539,7 @@ def build_schema_param(self, p_val):
         p_schema = {"type": p_type, "default": p_val}
 
         # Assume that false and empty strings shouldn't be a default
-        if p_val == "false" or p_val == "":
+        if p_val == "false" or p_val == "" or p_val is None:
             del p_schema["default"]
 
         return p_schema