doc: fix default value returned on getenv in man

Fix documentation of getenv modulefile command to describe that an empty
string is now returned when designated environment variable is not
defined and no default value to return is specified.

Author: xdelaruelle

doc/source/modulefile.rst

@@ -154,7 +154,7 @@ the *modulefile* is being loaded.
 .. mfcmd:: getenv variable [value]
 
  Returns value of environment *variable*. If *variable* is not defined,
- *value* is returned if set, ``_UNDEFINED_`` is returned otherwise. The
+ *value* is returned if set, an empty string is returned otherwise. The
  :mfcmd:`getenv` command should be preferred over the Tcl global variable
  ``env`` to query environment variables.
 
@@ -165,6 +165,10 @@ the *modulefile* is being loaded.
 
     .. versionadded:: 4.0
 
+    .. versionchanged:: 5.0
+       An empty string is returned instead of ``_UNDEFINED_`` when *variable*
+       is not defined and no *value* is set
+
 .. mfcmd:: getvariant variant [value]
 
  Returns value of designated *variant*. If *variant* is not defined, *value*