BUG: Replace in string series with NA (pandas-dev#32621)

Added condition for when to apply the na replacement

Author: chrispe

pandas/core/internals/managers.py

@@ -1934,10 +1934,11 @@ def _compare_or_regex_search(a, b, regex=False):
     is_a_array = isinstance(a, np.ndarray)
     is_b_array = isinstance(b, np.ndarray)
 
-    # Comparison with pd.NA is not possible. Replace it's value with np.nan.
-    # Based on the definition of isna: np.nan is equal to a pd.NA
-    a = np.where(isna(a), np.nan, a) 
-    b = np.where(isna(b), np.nan, b) 
+    # Replace all definitions of missing values (isna=True) to a numpy.nan
+    if is_a_array:
+        a = np.where(isna(a), np.nan, a)
+    if is_b_array:
+        b = np.where(isna(b), np.nan, b)
 
     if is_datetimelike_v_numeric(a, b) or is_numeric_v_string_like(a, b):
         # GH#29553 avoid deprecation warnings from numpy