fix bug for empty requirements

an empty set of requirements gives an empty requirement string which
was wrongly parsed to an CondaTarget with empty package name

This triggered a `conda search ''` which took so much memory that other
processes crashed.

The fix is to parse an empty list of CondaTargets for an empty target
string. In addition we check for empty package name during the creation
of the CondaTarget (to make this more easily detectable in the future)

lib/galaxy/tool_util/deps/conda_util.py

@@ -425,7 +425,7 @@ class CondaTarget:
     def __init__(
         self, package: str, version: Optional[str] = None, build: Optional[str] = None, channel: Optional[str] = None
     ) -> None:
-        if SHELL_UNSAFE_PATTERN.search(package) is not None:
+        if SHELL_UNSAFE_PATTERN.search(package) is not None or not package:
             raise ValueError(f"Invalid package [{package}] encountered.")
         self.capitalized_package = package
         self.package = package.lower()

lib/galaxy/tool_util/deps/mulled/mulled_build.py

@@ -527,8 +527,8 @@ def add_single_image_arguments(parser):
     )
 
 
-def target_str_to_targets(targets_raw):
-    def parse_target(target_str):
+def target_str_to_targets(targets_raw: str) -> List[CondaTarget]:
+    def parse_target(target_str: str) -> CondaTarget:
         if "=" in target_str:
             package_name, version = target_str.split("=", 1)
             build = None
@@ -540,9 +540,10 @@ def parse_target(target_str):
         else:
             target = build_target(target_str)
         return target
-
-    targets = [parse_target(_) for _ in targets_raw.split(",")]
-    return targets
+    if targets_raw.strip() == "":
+        return []
+    else:
+        return [parse_target(_) for _ in targets_raw.split(",")]
 
 
 def args_to_mull_targets_kwds(args):