fix(docs): remove newlines interpreted as test

When documenting mandatory arguments of a methods builder, it was
possible to document 'parts', which have a long multi-line description
with empty lines inbetween. This caused the indentation to be funny
and tricked rustdoc into thinking these are indented doc-tests.

Now we remove these empty lines, also hoping we dont encounter lines
with just whitespace in them. The latter would require a tiny update
of our regex.

src/mako/lib/rbuild.mako

@@ -6,7 +6,7 @@
                       rust_copy_value_s, is_required_property, organize_params, REQUEST_VALUE_PROPERTY_NAME,
                       build_all_params, rb_type_params_s, hub_type_params_s, mb_type_params_s, mb_additional_type_params, 
                       struct_type_bounds_s, METHODS_RESOURCE, SPACES_PER_TAB, prefix_all_but_first_with,
-                      METHODS_BUILDER_MARKER_TRAIT)
+                      METHODS_BUILDER_MARKER_TRAIT, remove_empty_lines)
 %>\
 <%namespace name="util" file="util.mako"/>\
 <%namespace name="lib" file="lib.mako"/>\
@@ -91,7 +91,7 @@ impl${rb_params} ${ThisType} {
         arg_prefix = "/// * `" + p.name + "` - "
 %>\
     ${arg_prefix}${p.get('description', "No description provided.")
-        | prefix_all_but_first_with(' ' * SPACES_PER_TAB + '///'  + ' ' * (len(arg_prefix) - len('///')))}
+        | remove_empty_lines, prefix_all_but_first_with(' ' * SPACES_PER_TAB + '///'  + ' ' * (len(arg_prefix) - len('///')))}
     % endfor
     % endif
     pub fn ${mangle_ident(a)}${type_params}(&self${method_args}) -> ${RType}${mb_tparams} {

src/mako/lib/util.py

@@ -150,6 +150,10 @@ def unindent_inner(s):
         return re_linestart.sub(' ' * tabs * SPACES_PER_TAB, s)
     return unindent_inner
 
+# filter to remove empty lines from a string
+def remove_empty_lines(s):
+    return re.sub("^\n", '', s, flags=re.MULTILINE)
+
 # Prepend prefix  to each line but the first
 def prefix_all_but_first_with(prefix):
     def indent_inner(s):