SCons: Fix Web build when compiler version isn't properly detected

Quick fix for #82585.

A better fix requires refactoring the way we detect the compiler version
to make it more reliable, and get a consistent output. But I prefer to
keep changes minimal for 3.x branches at this point.

Also set CI version to 3.1.39, which is what we use for official 3.6 builds
since 3.6-beta4.

.github/workflows/javascript_builds.yml

@@ -7,7 +7,7 @@ env:
   # Only used for the cache key. Increment version to force clean build.
   GODOT_BASE_BRANCH: 3.x
   SCONSFLAGS: verbose=yes warnings=all werror=yes debug_symbols=no
-  EM_VERSION: 3.1.45
+  EM_VERSION: 3.1.39
   EM_CACHE_FOLDER: "emsdk-cache"
 
 concurrency:

platform/javascript/detect.py

@@ -176,7 +176,7 @@ def configure(env):
     env["LIBSUFFIXES"] = ["$LIBSUFFIX"]
 
     # Get version info for checks below.
-    cc_semver = tuple(get_compiler_version(env))
+    cc_semver = tuple(get_compiler_version(env) or (3, 1, 39))
 
     env.Prepend(CPPPATH=["#platform/javascript"])
     env.Append(CPPDEFINES=["JAVASCRIPT_ENABLED", "UNIX_ENABLED"])